Hi @GabrielSantos ,
Based on my test, we cannot subscribe with external email address in a P1 capacity currently, it seems this feature does not formally rolled out yet, the 2020-3-2 is a expect date mentioned in the post. Once these features have formally rolled out , there will have a follow-up post and links to the public documentation.
Community Support Team _ Dong Li
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.