You Can't really do this dynamically very easily, but if you ware adding the form to a specific channel you could make use of URL parameters. But the issue is Teams tab doesn't support parameters currently. You could probably link the tab using website tab, to a powerapp directly and then pass in parameters to your app to achieve this but you might end up with a login prompt possibly on that website tab ( you would need to test that out).
check out this article on using parameters, you can pass the Name or the channel or whatever you want in the URL if the website tab will work for you, then you'll be able to do it with this.
https://blog.kloud.com.au/2017/10/17/how-to-pass-parameters-into-a-powerapp/
