AMAZON SP-API OAuth help for desktop application

I have a windows DESKTOP application that is listed on Amazon Developers Apps and uses Amazon MWS API and Amazon is requesting that we migrate to Amazon SP-API. The Amazon SP-API uses OAuth. I need to some assistance convert from MWS API to SP-API and implementing OAuth for my windows application.

When I'm editing my app on Amazon's App registration page I don't know what to put into the OAuth Login URI

How to support Amazon OAuth from desktop?

see attached