how do I get a test user's user ID?

[uncle_hunty]

I'm trying to test IAP using this method:
https://developers.meta.com/horizon/documentation/unity/ps-iap-s2s/#verify

I think I'm getting close, but when I use the curl command with "user_id=[test username]" it complains that I'm not using a valid user id.

{"error":{"message":"Parameter user_id: invalid user id: [test user's username]" ...

So how can I get the game-specific user id for a test user? I don't see any options to do so on the developer portal, and I haven't had any luck googling it. I would expect that id to be on the test users page on the developer portal for this game, but it's not.

[uncle_hunty]

I found a way to get a user ID, but it doesn't seem to work.

by exporting my test user list to csv, it DOES include a user ID:

...but when I try the curl with that user id it STILL gives me the same error that it's not a valid user id. Maybe that user id is the global meta user id and not the app-specific one that I need?

EDIT: yeah, that isn't the app-specific user ID; I added that user to a different project and exported the data from there, and it's still the same user ID.

[uncle_hunty]

Surely I don't have to add functionality to my app to fetch and display user ID, and then play the app as my test user. There must be a better way to get that user ID.