Is that block a recent thing? It might be different as for my on GSuite account I can add the name+addon@mydomain.com - it might just be a difference between the "public" Gmail system vs the Gsuite Gmail system. In which case, my question is completely invalid and feel free to ignore ;)
Note: This comes from my own developer account having 3 name+addon@ accounts live, and working with things like ApplePay etc for testing.