How to transfer an pre-release app (TestFlight only)

I wish to transfer my beta app (in TestFlight only) to another iTunes connect account. however, when running through the instructions online, I don't have the Transfer App option available. I've turned off TestFlight for Internal & External versions and my app has a status of "Ready for Submission". I want to ransfer the app to a company iTC account that I've created as the app is under my personal account and my developer membership is about to expire. Does anyone know how this is done? I've contact iTC support and have a case but have not been responded to.

Apple are playing with us. 2021 and we have same problem

How to transfer an pre-release app (TestFlight only)
 
 
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