Plex Custom Web App URL

[StiglitzWork]

In Settings/plex, one can define a custom plex Web App URL. e.g. plex.mydomain.com

With that, when I click on "Watch on Plex" (cf image) on a movie/tv show, I would expect overseerr to redirect me to plex.mydomain.com.
Which it does not do. I am redirected to plex.mydomain.com/#!/server/<an_id>/web/index.html.
This URL gets a 401.

[StiglitzWork]

Found out!
Add /web to your Plex custom WebApp URL
e.g. plex.mydomain.com/web