Displaying Docked form on Secondary Monitor

I have an app where I am docking my MainForm to either the left or right of
the Primary Monitor. How can I set the Forms current Monitor to be a
different one from the .Screens Collection ?

I want to be able to offer the user the use of moving this to dock on a
different monitor in a multi monitor setup, and find out what monitor it is
on before closing so I can save it and then restore it to the same monitor
on startup next time.

Thanks for any help or pointers
Martin