Public/Spamfilter/New-SpamfilterDomain.ps1

function New-SpamfilterDomain {
    Param(
        [Parameter(Mandatory)]
        [string]$DomainName,
        [Parameter(Mandatory)]
        [string]$Destination
    )
    process {
        try {
            $Credential = Get-SpamFilterCredential
        }
        catch {
            throw "Failed to retreive Spamfilter credential: $_"
        }

        try {
            $DomainName = [System.Web.HttpUtility]::UrlEncode($DomainName)
            $Destination = [System.Web.HttpUtility]::UrlEncode($Destination)
            $CompleteURI = "$($SpamfilterAPIBaseURL)/domain/add/domain/$($DomainName)/destinations/[" + '"' + $Destination + '"]'
            $Response = (Invoke-WebRequest -Credential $Credential -Uri $CompleteURI).Content
        }
        catch {
            throw "Failed to add domain to spamfilter: $_"
        }
        $Response
    }
}