Hi davis,
Huh, do we still have a hotkey in the manual for it?
Click View ->Render Border. Then drag with the mouse where you want your rectangle. This area gets rendered with your current chosen renderer then. Either when you set the viewport rendering to Rendered. Or switch to camera view, then mark a render border, then this area gets rendered when you hit F12.
To clear the render border use Clear Render border 
Kind regards
Reiner