If you already have an app, you’ll now find it (and the :env name) on the new Developer Dashboard here --> https://developer.twitter.com/en/account/environments
In my case, the beta environment and app shows up as env-beta. Once upgraded to a premium tier that offers more than one environment, you’re able to add an additional app if you like, and name the environment.
I believe that the new Dashboard / portal will enable environments to be renamed in the future, but at the moment a workaround would be to delete, create a new environment, and associate the same app. This is new functionality today, so please excuse me if I’m not 100% correct on this detail as I’ve not yet had to go through it myself. I can say with certainty that your beta app should appear in the Dashboard now.