Forms authentication can use many forms. The authentication applies to the entire application (using a typicaly setup). Once you are authenticated you can access any form in the application. After you have verified login you can redirect to whatever form you want.
Here are the two typical methods used with forms authentication:
Use this method to issue the authentication cookie and automatically redirect to whatever page the user was trying to access when they were forced to go to the login page to authenticate.
Use this method to issue the cookie without automatic redirect, then issue your own redirect to whatever specific page you want.