Hold shift to enable line drawing, press Space to snap to right angles
< Feedback on Skribbl Line Tool
You can try skribble shapes tool -- It has 45 deg lines and much more feature https://greasyfork.org/en/scripts/438046-the-skribble-shapes-tool-10-features
The script works perfectly! The only complaint I have is that using space only allows for 90-degree lines, not 45 or 22.5 ones. Works exactly as it said it should.