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 . Their wiki's have no powershell examples :(
Thanks