Overseerr is the app I use to manage requests. You can get to Overseerr at https://requests.capntimbo.com/
Once there, click "Sign In" under "Use your Plex account". Use the same credentials you use to sign into Plex.
Once signed in, you can search for the show or movie that you want to request using the search bar at the top.
After you select a show or movie, you can click the request button on the right. If it's a show, you can select the specific seasons that you want.
When you submit a request, it sends me an email. If you're really desparate to get the show or movie, text or call me after you submit it.
You can request shows or movies that haven't been released yet. If I approve the request, they'll be downloaded as soon as they are available.