Access WebService in WebApplication with Forms Authentication

I have a WebApplication that uses forms authentication. In an subfolder (fe. webservices) i want to include some simple WebServices. Those WebServices should not use a separate Logon Procedure, perhaps use the already implemented Authentication of the WebApplication.
So far so good, i could access the WebServices when allowing the anonymous access in the webservices folder. But how can i access when the anonymous access is disabled?
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