I never use a full-screen print screen. On windows I usually make a shortcut for the snipping tool and mac has their feature built-in (something like ctrl+shift+f4).
Keyboard shortcuts shows that print-screen button is mapped to xfce4-screenshooter -f. The -f obviously means full-screen and I know this tool has the feature for selection. So I look up the man page for the tool.
-r or –region is what I am looking for so on keyboard>shortcuts I just swapped the -f for a -r