Powershell SOAP for ServiceNow

I am trying to use Powershell SOAP to create a ticket in ServiceNow. I have valid XML that I have successfully tested with the SoapUI tool using basic auth. But when I try to post that same XML to ServiceNow via Powershell I get an error. I'm not sure if its a problem with the auth header (same creds in both SoapUI and PS) or something else because I'm getting a 500 error from the post, which doesn't sound auth related.

Using PS v4. Long time powershell user. First time doing SOAP and scripted interaction with ServiceNow. Their wiki's have no powershell examples :(