Hello mwiseley,
I would keep it first simple, I use -TLS and it knows what port to use.
Get-Content d:\comps.txt | Invoke-AMTPowerManagement -TLS -Username XXXX -Password XXXXX -Operation:PowerOn
This line works for me
Once you get this working you can go a step deeper with commands like write-AMTcredential, read-AMTcredential but first keep it as basic as possible, like this you see quicker where the problem in a script is
Jan