Quote from Steam FAQ link:
To do this, on the DCS website, on the profile page, find a special Steam account block
please use the link: https://www.digitalcombatsimulator.com/en/personal/profile/
1) You need to link the Steam account to your DCS account.
After clicking on the "BIND" button, you will be redirected to the Steam website, where you must log in and agree to share publicly available information from your profile with us.
2) After logging in to the Steam site, you return to our site and, if successful, the "GET LICENSES" button will be available.
Click the "Get licenses" button.
3) As a result, you can obtain a list of the products whose licenses that:
1. Already have a DCS account (Steam or not Steam)
2. Can be transferred to your account from Steam
3. Can not be transferred because they are in different types of protection
4) Click the "Transfer licenses" button.
As a result, the licenses available for transfer will be bound to the DCS account.