I needed to highlight some text in my screenshots and I did not know how to do it with the default screenshot app. Flameshot is a solution. Install it. If you need to replace the PrtScr shortcut do the following:

1. release the PrtScr binding by this command

gsettings set org.gnome.settings-daemon.plugins.media-keys screenshot ''

2. Go to Settings->Devices->Keyboard and scroll to the end. Press + and you will create custom shortcut.

Enter name "flameshot"

command "/usr/bin/flameshot gui"

set shortcut to PrtScr (print)

That is it. Next time you push PrtScr flameshot will be launched.