Well, I thing it was pure luck. Or, at least, sort of luck, lol. I can’t remember exactly but, I simply opened a beta page and I thought it was kind different. I clicked the dropdown menu and there it was: the “Request movie poster” option, along with other hiddden options in the new beta page.
I tried to redo the steps and went to the classic movie page, than clicked in a movie page where there wasn’t a poster and clicked for “Add a poster” but it lead to the new beta page.
Then, with the two tabs opened, I saw there was such a thing as “oldbeta” and the way to get there can be done by simply adding “old” to the beginning of the url.
(I think that the old beta pages are connected with the url pages in the icoming messages for lists updates. I cannot confirm right now because I deleted the most recent message for this in my inbox, but I almost certain it came from that)