PowerShell will allow you to pass any ScriptBlock object to a .NET method which expects a delegate or Func/Action object. In this case, RemoteCertificateValidationCallback is passed 4 arguments, and expected to return a bool, so you could do this:
You don’t have to use a param block if you don’t want to (in which case you would refer to $args[0] through $args[3]), and you don’t have to assign the delegate to another variable before calling your constructor, but those things can often make the code easier to read.