Plex integration setup

I am having difficulties in setting up plex via the integrations page.
Allright, I have setup my plex server and everything is fine. ( I havent enabled the remote access, because I dont want it - even though for the purpose of this setup, I enabled it for a brief moment.)
Now I am trying to setup the integration and when the authorization website opens, I get an error 404.
Here is what I do step by step:

I am selecting the plex integration and I am clicking submit.
Opera Snapshot_2020-04-13_214120_192.168.1.20

I am clicking sign in.

And now this pops up:
SharedScreenshot

The URL is my DDNS domain. I havent port forwarded anything. Should I? What port and why? Since I am providing my email to sign in, it was supposed to work without port forward…

Make sure your base_url is set correctly: https://www.home-assistant.io/integrations/http/