Hi @samliew ,
I just can’t thank you enough for your help! Your solution worked like a charm! I used the second API Key authentication example you gave me. Thank you also for the tip about the Enterprise plan - that will make things easier too.
I’m going to expound on your solution with a few screenshots in case anyone else, who is trying to set up webhook authentication and also test with Postman, comes across this post.
Thank you again for your help and for getting back to me so quickly! You saved me a lot of time and extra frustration - I was already careening toward frustration when I reached out! 
What a great resource this community is!
-Kari
Here is my Postman API Key authentication setup:
In the webhook, I toggled “Show advanced settings” on and made sure “Get request headers” was set to “Yes.”
Here’s where I put the “Key” (arbitrary key name) and “Value” (arbitrary password) from Postman into the filter you showed me how to set up:
This is what the scenario looked like when the filter did its job (password didn’t match, authentication failed - filter icon has “0” badge, and payload did not pass to the next module.)
This is what the scenario looked like when the password matched (authentication passed, and payload was passed to the next module.)
