You may need to post some of your code for us to be able to help you as it would depend on the methods you are using. However, if you look at the LogonForm web part (under the memberships folder), you will see that Kentico Redirects the user once they are LoggedIn. They use the standard ASP.Net login control and hence have attached to the events that associated with that. Therefore, if you use a custom login control you will need to see the method to add the URL Redirect to (obviously should be easy as it would be after you have validated that the user is LoggedIn).