Authentication when my web service is being request?

I am using .NET roles and membership model to manage user accounts in my system back-end.
If the request is come from within my web-app, I can authenticate through form.
But if the user is requesting from else where, maybe their own apps, how to I do authentication?
attach username and password (GET/POST) and then secure the line by SSL?  is that what I should do?
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