The Mailroom streamer frog secret codes are seven plain words: lion, case, mouse, zeke, ninja, vanilla and thor. Type one on your keyboard while you play and a special streamer frog appears. They are typed as plain words with no prefix, and they work whether or not you are streaming.
The Mailroom streamer frog secret codes: what they change
The frog that appears is a variant of the ordinary office amphibian rather than a different kind of creature, so a code changes the frog you are holding instead of unlocking a separate mode or a new area. That is the whole effect: the amphibian on your bench is the special one for the rest of the shift.
Because the variant is still an office amphibian, the same bench routine applies to it as to any other frog: it is dissected, or pet, on the bench to produce raw material for the upper floors. The full frog job is in frogs and crafting.
Because the trigger is a word on your own keyboard, the codes are worth keeping in a chat window or a notepad for when a viewer asks about them mid-stream.
Letting chat do the damage
The same update that brought the frog codes also brought Twitch chat interactions, where your viewers type commands of their own and make your shift harder. Those commands are listed in Twitch chat commands.