I finally figured it out. The 8 digit numeric-only code is actually one generated by amazon apparently as a substitute for the seven digit alpha-numeric code displayed on the hosting device. I only stumbled on it by accident and perseverance because it isn't offered as a choice unless you find it in a very particular way.
When I typed "https:\\watchparty.amazon\1x1x1x1" where 1x1x1x1 is the 7 digit alpha-numeric into a browser on my PC (tried both MS Edge and firefox), I was redirected to the Join Watch Party page as expected but no alternative code was offered. (NOTE: you must leave off a trailing backslash as the code is a parameter)
HOWEVER, when using a browser on my phone and the same address, I got an whole new section with a heading "Other ways to join" that describes the following:
1. On your TV, open the Prime Video app.
2. Select My Stuff
3. Select Join a watch party and enter the Watch Party code.
12345678 <- the code you will need to enter the party through the the Roku Amazon app
I have no idea why this section isn't available through a browser on a PC and only through a browser on a phone. Perhaps Amazon wants it to be difficult through Roku to support their competing Fire TV Stick.