Thanks for your report 
Hm. The manual should not indicate any hotkey. So that’s something to invest. Bforartists relies at the graphical user interface. Hotkeys is something that Users have to set up when it is required.
The feature that you search for is not called Render Region, but Render Border. And can be found in the View menu. like in Blender. The hotkey is removed in the Bforartists Keymap. When you use it often, then simply assign a hotkey to it.
Kind regards
Reiner