If the code came with hardware (like a controller or console), look for a voucher included in the packaging. How to Redeem Your Code
If you want to force your friends to join your crew (metaphorically, of course), buying a key code is the easiest way. You purchase the code, receive it via email, and then simply forward that code to your friend. They redeem it, and you have a new shipmate. This bypasses the complicated "gift" restrictions on Steam or Xbox. sea of thieves key code