How do I put authentication on a REST service for Cross Platform?

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ASP.NET

REST

WebAPI

I am trying to create a REST service that will be used on Android, iPhone and Windows Phone. We need the calls to be behind password wall so we will have to issue you the login information.

I found this: Cross Platform Authentication With ASP.NET Web API[^]

It seems to do the trick nicely at first putting it behind a wall that requires HTTPS to function and with a nice encryption using your IP address to help secure it a little better. I got it added to my project but I can't for the life of me figure out how to pass across the user information correctly so I can't use it. I have looked through the content of the different websites this was posted on but I haven't seen any activity in nearly a year. I was wondering if anyone out there knew a solution that would work for cross platform?