Using search, look for screen, click on the highlighted Save a screenshot to Pictures (currently set to Printscreen (PrtSc keyboard key.)
Press the backspace key on the keyboard and it will disable the mapping.
Name it something you will remember, and for the command path type in the path to Flameshot.
You will most likely want the graphical user interface to launch so add
/usr/bin/flameshot gui. Click on the X to close the window out.
Pressing the print screen (PrtSc) key on your keyboard should now launch Flameshot.