Firstly you need to connect Maintainerr with your Plex server:
1. Click Authenticate with Plex.
2. Now in the pop up window, log in to your Plex account.
3. Reveal and choose the server you want to get connected with Mainainerr from the list.
Click Save Changes, then you can use Test Saved (in this order otherwise an error will occur).
Overseerr
Hostname or IP: Overseerr URL
Port: 443
Api key: taken from Overseerr Settings
Click Save Changes, then you can use Test Saved (in this order otherwise an error will occur).
Radarr
Hostname or IP: Radarr URL
Port: 443
Base URL: leave empty
Api key: taken from Radarr -> Settings -> General
Click Save Changes, then you can use Test Saved (in this order otherwise an error will occur).
Sonarr
Hostname or IP: Sonarr URL
Port: 443
Base URL: leave empty
Api key: taken from Sonarr -> Settings -> General
Click Save Changes, then you can use Test Saved (in this order otherwise an error will occur).