Hey Mariusz, I managed to add the community beta, but the Start9 beta comes up as an invalid URL, thanks for the help BTW 👍 
It is already in the BETA Registry:
https://marketplace.start9.com/mutiny-wallet?api=beta-registry.start9.com&name=Beta%20Registry
Discussion
just use: https://beta-registry.start9.com/
I got it now, thanks a lot 👍