I did a quick search in the forum and saw someone suggesting putting N/A in that field, and it seemed to have worked.

@seyman said:
Creating API-key, what should go in the Application-URL?
I am trying to create an API-key so that I can test out fetching a list of movies to my test project (that is not published online, but just a local project).
So now I wonder what I should put in the field "Application-URL"?
Since my site is not online, what should I add here?
It seems I can't leave it empty, and I see no help-icon in the section, so I am at a loss here.

If you are an APP developer but still do not have a URL for your software,
or if you are a teacher teaching your class how to use the API feature,
or will use it in their own studies,
or in short, do not have a URL,
just inform N/A, not applicable or not available or no answer.

this solution is not working anymore please suggest something else.

Not working. N/A or "n/a" does not work.

Cannot proceed. Impossible to Submit. Impossible to get API key. Impossible to use with UGREEN NAS media library. ....please advise.