Sorry about the late reply, guys!
Yes, I was able to test IAP for standalone watchOS app. You need to run you app on a real device (not the simulator), and you need to log in with a Sandbox-account on the bottom of Settings --> iTunes and App Store on your iPhone. Please note that you currently need to create a new Sandbox-account for each test purchase, but from what I remember from WWDC this should be better in watchOS7 with the improved StoreKit - https://developer.apple.com/documentation/storekit/in-app_purchase/testing_at_all_stages_of_development_with_xcode_and_sandbox.
strandlie
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It turns out that XCode does not support adding the In App Purchase Capability for independent watchOS apps explicitly - but you don't need to. When you create your App ID in your developer profile it supports IAP's and it works.
It is here, in the new betas of XCode 11.4 and WatchOS 6.2. I'm having some trouble getting it set up correctly. I can't add the IAP Capability to my WatchKit Extension Target, even though I have set the bundle identifier and the provisioning profile correctly. Any tips here?
This problem was fixed for me in the new XCode Beta.