sending user credentials on subsequent requests in web api basic authentication

I'm working on a RESTful mobile application using ASP.Net Web APIs. I am inclined to use Basic authentication. I have followed some articles which gave me good start. I understood that "username:password" will be encoded and added to "Authorization: Basic " and sent it as header on subsequent http requests after authentication. But I have only one thing missing here is,
1.How do I send this "username:password" encoded string on every http request without using session or cookies
In Forms Authentication a cookie will be set. In membership User Principal will be set.
All http requests to web api comes from ajax and I need to set authorization header in beforeSend. But where do I get "username:password" encoded string from, on every page ajax request