Hi @ChrisM-4360 ,
The Permission Request XML box is always empty, even when you have previously added permissions.
Unfortunately it works this way,when you add permissions, they are assigned to the app, but are not shown later.
I suggest you can store the the Permission Request XML with App Id, App Secret to a txt file in your localhost so that it can be used in later date.
Another option is that we can override the Permission Request xml in Appinv.aspx when you have the App Id with Lookup button.
Here is a same question in StackOverflow for your reference:
App's Permission Request XML not filled when accessing appinv.aspx again