I capture a selected region using Shift + PrtScr frequently, and I use red box in region mode. I have it set to the dimensions I want, and I just drag it into position.
At that point I would like to avoid the capturing steps of (1) having to right click the bar at the top of the box, and (2) having to choose "Capture Object Now" from the drop-down menu. I have tried every hotkey suggested and anything else I can think of, but have not found a way to eliminate one or both of these time-wasting steps when I am ready to capture the region I've chosen.
I've attached an image of what I see on my screen when I have set up a selected region capture as described.