John was asking a very straight-forward question in order to understand what you're trying to do. The first thing before any of us can offer suggestions.
Click on the mask 'name' line in the Effects Control Panel (ECP) and the mask will appear, you can select vertices and work on them.
I'll add ... being rude back to another user who's helping simply out of their trying to be helpful isn't really a best-practice. We're nearly all users here, as this is peer-to-peer "help". Not obvious to those just starting out ... as well, not much of PrPro is obvious just starting out either.
This is a pro-level, very complex and complicated app. In a pro app, there simply is not near the auto/ease-of-use found in the prosumer apps. So getting some training coming into it is pretty much needed by anyone.
I found the Premiere Pro YouTube things out on the web not always that helpful, and then found what's now LinkedInLearning's tutorials. And realized quickly that a month or so spent going through several of their full courses with included media was a vastly better way to actually get doing something than just trying to "poke the box".
And I normally start anything by just poking. PrPro ... ain't an app suited for that until you've got used to the basic way it works.