Check your mail box and find the purchase confirmation e-mail from Bohemia Interactive. A Steam code for the purchased game or other product is inside.
You can, of course, login to your Bohemia Store Account to check your purchases and codes. When you login to your Bohemia Store account (https://accounts.bistudio.com/) with your e-mail and password, you can review your previous purchases. Click on the "Purchases" button and then "Details"
Learn more about code activation on Steam on this page https://support.bohemia.net/store-bistudio/activation/230