How to regain Focus?

So I am working on custom window... All works as long as I use either mouse or keyboard. Issue is when I use the mouse and then go back to keyboard.

Basically I have one primary button I want to have the main focus and from that point I can navigate to and from. What I am looking for is when I take the mouse and hover to a "dead" area where there are no controls, then when I press up or down on the keyboard I want it to set the focus back to this primary button. Is there a clean correct way to do this. I know I can hack it by putting a fake hidden button the size of the screen with the up down left right set to go to the primary button, but i am hopeing someone know a better way.


Thanks.