Overload error for method "System.Net.CredentialCache.Add" in PSCore 6.2.3

Question

I have the following script to perform an availability check for a password-protected localhost site:

Function CheckAvailability ($Url, $Credentials)
{
    $WebClient = New-Object System.Net.WebClient
    [System.Net.CredentialCache]$CredentialCache = New-Object System.Net.CredentialCache
    $CredentialCache.Add($Url, "Basic", $Credentials)
    $WebClient.Credentials = $CredentialCache

    If ($WebClient.DownloadString($Url))
    {
        Return $True
    }
    Else
    {
        Return $False
    }
}

Function Main ()
{
    $Url = "http://localhost/resource"
    $Credentials = New-Object System.Net.NetworkCredential("admin","password","") #UserName, Password, Domain
    CheckAvailability($Url, $Credentials)
}

Main

If I run the script, I get the following error

Cannot find an overload for "Add" and the argument count: "3". At D:\WebSiteTest.ps1 + $CredentialCache.Add($Url, "Basic", $Credentials) + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + CategoryInfo : NotSpecified: (:) [], MethodException + FullyQualifiedErrorId : MethodCountCouldNotFindBest

I've read that the method add and it should support 3 or 4 parameters (https://learn.microsoft.com/en-us/dotnet/api/system.net.credentialcache.add?view=netframework-4.8). Can someone tell me, what the issue is here? I am running Powershell Core 6.2.3.

I appreciate any suggestions.