Overseerr is a request management and media discovery tool built to work with your existing Plex ecosystem.
Once you've enabled it in your account, launch the app via your dashboard. You'll need to perform the initial setup, signing in with your Plex account, and connecting to your apps of choice (see Interconnecting-Apps)
Relevant details below:
plex
32400
radarr
7878
(get api key from UI at Settings -> General)
sonarr
8989
(get api key from UI at Settings -> General)