Hi Jake,
You should be able to find the license key on the back of your manual if you bought a hard copy of the game. If it's a digital copy from the Ship Simulator Website, then you should have been given a key which will be available to you on your account on the www.shipsim.com website upon signing in.
With regards to a Steam purchase, I'm not sure - maybe someone else could help if you bought the game there. I can only assume that you would be issued with the key on Purchase, similar to buying the game from shipsim.com
Kind Regards,
Jack.