How to submit IAP for TestFlight beta?

Hi there! I've got an app to submit to TestFlight for a beta. It has IAP which I'd like to submit also so users can test the flow, obviously without being charged. It's completely possible, as evidence by the actual Apple website mentioning the in-app purchases down the bottom of the page here:

https://developer.apple.com/testflight/

I've added the IAP and its information in ITC, and it says Ready to Submit next to each one. When looking at individual details, it says:

"Your first In-App Purchase must be submitted with a new app version. Select it from the app’s In-App Purchases section and click Submit. Once your binary has been uploaded and your first In-App Purchase has been submitted for review, additional In-App Purchases can be submitted using the table below."

So I have no idea what to do from this point. I don't want to submit an actual binary to full App Store review, because it's still in beta. When I submit a new binary for TestFlight review, it says nothing about IAP.

Anyone been through this process and know how to get IAP approved without actually submitting the full binary for review? Thanks!