How do you get the UserID from an authenticated SharePoint WebPart?

I'm building a SharePoint WebPart, and using iMIS 15.1.1

SSO is working, so the SharePoint client is authenticated.

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How do I get the iMIS UserID?

Is there a method on the MembershipWebService.asmx that can do that for me?

If there is no suitable method available, please suggest an approach!

 

Thank you,

~ Boris