Hey there @gary.brook
No matter how good the person that is going to provide you with help is, the context you provided simply doesn’t have any helpful information apart from the thing that it’s a Mac App and that you’re trying to implement Resource Owner Password grant No steps to reproduce, links to the quickstart you used and anything.
What stack of ours have you used? What docs of ours have you followed? At what step have you started receiving 403? Etc. etc
We do not have any quickstart for Mac apps, I’m guessing you built the UI yourself and now wants to authenticate hitting the right endpoint of our API (that’s my wild guess). Then you need to use our Authentication API: