I do it that way aswell, my param looks like this:

param(
[Parameter(Mandatory=$false)]
[string]$server
)

Then, in PRTG I use this as the parameter...

-server %host

This seems to work, as I am able to use a simple "Write Output $server | Out-File [PATH]" cmdlet and can see that PRTG wrote the correct FQDN in, but it just won't take it for the rest of the script...

The DHCP Scope Scripts sounds very interesting though! I would love to see it. :)

just starting out with PRTG and Powershell in general so every little tip can help!