Param ([Parameter(Mandatory = $false)] $UserID = $UserID)

DynamicParam { if ($PSBoundParameters['UserID']) { $UserID = $PSBoundParameters['UserID'] }
               else                              { $UserID = read-host "UserID"           }
               # ...    
             }

Param
(
    [Parameter(Mandatory=$true)] [ValidateNotNullOrEmpty()] [ValidateLength(1, 20)]
    [string] $UserID,
    
    [Parameter(Mandatory=$true)] $Domain
)

DynamicParam
{
    $ParameterDictionary = New-Object System.Management.Automation.RuntimeDefinedParameterDictionary

    if ((-not $PSBoundParameters['UserID']) -or ([string]::IsNullOrEmpty($UserID)))
    {
        # One can't abort straight from within DynamicParam, so just mark it and leave.
        $IsDynamicParamOK = $false
        return $ParameterDictionary
    }
    
    # Else: Continue with the normal code path, because everything was OK.
    # ...

    $IsDynamicParamOK = $true
    return $ParameterDictionary
}

End
{
   if ($IsDynamicParamOK -eq $false)
   {
       Write-Warning "Please provide the UserID directly on the command line: $($MyInvocation.MyCommand.Name) -UserID ..."
       return
   }

   # Else: Continue with the normal code path, because everything was OK.
   # ...
}

[CmdletBinding()]
Param
(
    [Parameter(Mandatory = $true)]  $Domain,
    [Parameter(Mandatory = $false)] $UserID = $UserID, # See 'Gotcha 2'.
                           [switch] $Optional
)

DynamicParam
{
    if ($PSBoundParameters['UserID']) { $UserID = $PSBoundParameters['UserID'] }
    else                              { $UserID = read-host "UserID" }

    if ([string]::IsNullOrEmpty($UserID)) { throw "`r`nThe UserID may not be empty!" }
    if ($UserID.Length -gt 20)            { throw "`r`nThe UserID may not exceed 20 characters!" }
    
    $IsService  = $false
    $IsExternal = $false   

    # 1. Define the parameter attributes with the ParameterAttribute object.
    $GeneralParameterAttributes = [System.Management.Automation.ParameterAttribute] @{
            Mandatory         = $true
            ParameterSetName  = '__AllParameterSets'
            HelpMessage       = "Help Message"       # For interactive help: '(Type !? for Help.)'
    }
        # Alternative for setting the above up:
        #   $GeneralParameterAttributes = New-Object System.Management.Automation.ParameterAttribute
        #   $GeneralParameterAttributes.ParameterSetName = '__AllParameterSets'
        #   $GeneralParameterAttributes.Mandatory        = $true
        #   [...]
    
    # 2. Create a RuntimeDefinedParameterDictionary object.
    #    (Note: During this sample code, these kind of dictionaries will be created/combined multiple times...)
    $Dictionary_Common = New-Object System.Management.Automation.RuntimeDefinedParameterDictionary
        
    $DP_Description = New-Object System.Management.Automation.RuntimeDefinedParameter('Description', [string], $Collection)
    $Dictionary_Common.Add('Description', $DP_Description)

    # 3. Create an Attribute collection object.
    $Collection = New-Object System.Collections.ObjectModel.Collection[System.Attribute]

    # 4. Add your ParameterAttribute to the Attribute collection.
    $Collection.Add($GeneralParameterAttributes)

    if ($UserID -like 'svc*')
    {
        $IsService  = $true
        $EmployeeID = "ServiceAccount"

        # (same as step 2...)
        $Dictionary_ServiceAccount = New-Object System.Management.Automation.RuntimeDefinedParameterDictionary
       
        # 5. Create the RuntimeDefinedParameter specifying:
        #    - The name of the parameter.
        #    - The type of the parameter.
        #    - The Attribute collection object you created in step 2.
        #   (This will be repeated and extended multiple times in the code below...)
        $DP_Name = New-Object System.Management.Automation.RuntimeDefinedParameter('Name', [string], $Collection)
        
        # 6. Add the RuntimeDefinedParameter to the RuntimeDefinedParameterDictionary.
        #   (This will be repeated multiple times in the code below...)
        $Dictionary_ServiceAccount.Add('Name', $DP_Name)
        
        $Dictionary = $Dictionary_ServiceAccount
    }
    else
    {
        # (The lines below are just specializations and extensions of the steps that came before...)
        
        # Copy a dictionary and extend it:
        $Dictionary_RegularAccount = $Dictionary_Common

        $DP_GivenName = New-Object System.Management.Automation.RuntimeDefinedParameter('GivenName', [string], $Collection)
        $Dictionary_RegularAccount.Add('GivenName', $DP_GivenName)
        
        $DP_Surname = New-Object System.Management.Automation.RuntimeDefinedParameter('Surname', [string], $Collection)
        $Dictionary_RegularAccount.Add('Surname', $DP_Surname)
        
        $DP_EmployeeID = New-Object System.Management.Automation.RuntimeDefinedParameter('EmployeeID', [string], $Collection)
        $Dictionary_RegularAccount.Add('EmployeeID', $DP_EmployeeID)

        if ($UserID -like 'uext*')
        {
            $IsExternal = $true

            $Dictionary_ExternalEmployee = $Dictionary_RegularAccount
                      
            $Manager_Attributes = New-Object System.Management.Automation.ParameterAttribute
            $Manager_Attributes.ParameterSetName = '__AllParameterSets'
            $Manager_Attributes.Mandatory        = $true
            
            $Manager_Collection = New-Object System.Collections.ObjectModel.Collection[System.Attribute]
            $Manager_Collection.Add($Manager_Attributes)
            
            # In this case, the parameter gets an alias:
            $Manager_Alias = New-Object -TypeName "System.Management.Automation.AliasAttribute" -ArgumentList 'Xyz'
            $Manager_Collection.Add($Manager_Alias)
            
            # ... and can be empty or null:
            $Manager_AllowEmptyString = New-Object -TypeName "System.Management.Automation.AllowEmptyStringAttribute"
            $Manager_Collection.Add($Manager_AllowEmptyString)
            
            $Manager_AllowNull = New-Object -TypeName "System.Management.Automation.AllowNullAttribute"
            $Manager_Collection.Add($Manager_AllowNull)
            
            $DP_Manager = New-Object System.Management.Automation.RuntimeDefinedParameter('Manager', [string], $Manager_Collection)
            $Dictionary_ExternalEmployee.Add('Manager', $DP_Manager)
            
            $Dictionary = $Dictionary_ExternalEmployee
        }
        else
        {
            $Dictionary_Default = $Dictionary_RegularAccount
            $Dictionary = $Dictionary_Default
        }
    }
    
    # 7. Return the RuntimeDefinedParameterDictionary object.
    return $Dictionary
}

End # See 'Gotcha 1'.
{
    write-host "------------------------------------------------------------"
    write-host "UserID: $UserID"

    if ($IsService)  { write-host "-> A Service Account" }
    if ($IsExternal) { write-host "-> An External Employee" }

    if ($PSBoundParameters['GivenName']) { write-host "GivenName: $($PSBoundParameters['GivenName'])" }
    if ($PSBoundParameters['Surname'])   { write-host "Surname: $($PSBoundParameters['Surname'])" }
    if ($PSBoundParameters['Optional'])  { write-host "Switch '-Optional' is active." }
}