Edit: important, ingame visit the Hollvania Mall and go where Arthur is and click on the phone right behind him (should be active the ‘hang out’ option).
Also don’t forget to go behind there stairs where you can find a hole in the floor that leads to a vending machine, click on it for a Drippy glyph.
Then login on the Warframe website and go here:
https://www.warframe.com/1999/doktorentrati
(You should be able to click on the end option)
and enter this codes with the same order
HOHHLKLLKOKO OHLKKOOLLHHK
KOOKLHLKOHLH OLOKKOKLHLHH
LLOHLHKKOOHK LKLOKHHOLHOK
This will give you a code for a poster ingame
I think that phone and mobile in general doesn’t work (it say that the window is too small to work). On my Desktop everything was fine (click start and on the top right appears a prompt where you can enter the sequence)
Was doing it all on PC. Kaldo was correct, this post was missing steps for someone who’s been ignoring the ARG. Took care of it and now have the poster.