[feature request] modify PR description

[andekande]

I'd like to be able to define a rule that sets the description of the PR, if it is currently empty. That will later be used as the default merge commit message.
It would be very nice if I could control whether I want to replace or append to the description.
The API doc states that write permissions to repo are required for this. Is this the case for a GitHub App?

https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#update-a-pull-request