Take this with a heaping spoonful of salt because I haven't actually done any work with user authentication.

But assuming the user has registered elsewhere (say on your website) and set up a username and password, I guess maybe you could prompt them for their info with a Keyboard node, then use the ifUrlTransfer function SetUserAndPassword(user as String, password as String) to do HTTP digest authentication of the user. Then after that perhaps you could save a token to the registry with their user info to verify that they have already logged in, so that they don't have to log in again every time they open the app up, and you can just pull the info from the registry every time they open the app to repeat the authentication (to verify their info hasn't changed/their account is still open)? And if the info you've saved in the registry ever stops working to authenticate the user, you prompt them to enter it again just like you did the first time they opened the app.

But again, I don't know. This is just a guess and it's likely not the way you're supposed to do it.