Very simple trophy to do, can do this solo or in coop.
To do this, you have first to create a Shift Account online. You can do this here : https://shift.gearboxsoftware.com
Once that's done, link it through your PSN account. If your game is ON, restart it. You can enter a Golden Key code either in the main menu or in-game.
Just click Start > Social > > > Shift Code.
Many codes are in circulation online, i suggest you take this website here : http://orcz.com/Borderlands_3:_Golden_Key
to find all the key.
When your code is entered and accepted, go to Social again and go to mail. You should have a mail with a golden key. Just click X on it and will have it in your inventory.
Then go to Sanctuary, find the Golden Key box and open it. The trophy should pop a few seconds after.