A text entry box will appear where you can enter codes like MANIFEST (to get 100 Fabrication Kits) or DREAMLAND (to unlock Nightmare mode).
Press the "T" key on your keyboard to open the global text chat and type your message. call-of-duty-black-ops-3-game
For more on how to chat or find hidden text in Call of Duty games, check out these guides: A text entry box will appear where you