I don't know how to give another developer access to create a SharePoint list

Hi!

I need help trying to get a developer a client id and client secret. He has built a web-based program, and we want information entered into that program to be logged in a SharePoint Library.  

I don't presently have access to the app. I don't really know where to start. Can someone please tell me what I need to do?

I receive the error below (Invalid App Domain).