If anyone is encountering problems to configure NetNewsWire to fetch RSS Feeds from your (self-hosted) FreshRSS, some tipps:
Screens on the MacOS and iOS Apps may be different, but the fields are the same.

Login
Login: username (the username you use for your FreshRSS login)
Password
Password: API-Password for your User. API Password must be set first in FreshRSS configuration.
Open FreshRSS in your Browser
A) Enable API Access in Settings > Authentication

B) Set API Password at Settings > Profile > API management

Use this password when setting up NetNewsWire.
UrI/Path/API Url
API Url: Server and Folder you have FreshRSS installed at + API Path
If you have FresshRSS installed into the homefolder then it’s
https://example.com/p/api/greader.php
If you have FreshRSS installed in a subfolder it’s something like
https://example.com/subfolder/p/api/greader.php
Do not forget the /p/api/greader.php