Hey
I might have missed something but can't you just edit the "expiration": null line in you HTTP request to make the permission expire...

As per documentation, the expected value is a string with format: yyyy-MM-ddTHH:mm:ssZ
So basically just replace null with the expression addDays(utcNow(), 30)
This should make it so the created permission expires in 30 days after the flow ran.
cheers