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