Hi Tom,
this would be the way to place the protective diode:
This could be a 1N4148 which is often used to protect optocoupler ICs.
For the Datasheet follow this link:
https://www.vishay.com/docs/81857/1n4148.pdf
I'm completely with our fellow members:
- You may mess up your multimeter and probably interfere with electrial safety.
- Today lots of multimeters at decent cost have an integrated continuity testing.
There are many tutorials in the net how to build your own tester. However you (and the intended use) may have to decide if a very simple solution is satifsfying your needs or if you want the device to have a (more or less) well defined resistance threshold.
Good luck!
ec2021