Please note that, in the following, for security reasons I have replaced actual identifying information with placeholders e.g. replaced the first part of our SharePoint domain name with mysharepoint as in mysharepoint.sharepoint.com
The first form I mention above - appregnew.aspx - is also asking for an App Domain and a Redirect URI.
I cannot find any guidance anywhere on what these values really mean and what they should be set to.
Do they relate to the domain in which our C# program resides?
If so, how would we make this happen when our C# program makes the request to Sharepoint i.e. how do we associate a domain with our C# program in such a way that Sharepoint will identify it when our C# program is making a request to Sharepoint?