I am not sure how you want this to work but here is what I have in mind:
1. Place the "Login" public control on some page that is accessible to all users.
2. Open the controls "Edit" and in the "Misc" section click on the "Edit
3. Open the "Behavior" section and in the "DestinationPageUrl
" select one of the non public page.
This way a user can go to the public page -> then login and from there he will be automatically navigated to the non public page.
The reason for this controls to exist is that sometimes people place "Firewalls" to deny users form trying to login inside the CMS's admin section. Also the "Login" control can be styled more easily and it is much more handy.
the Telerik team