Let's say you purchase a licence and specify you main domain as "example.com". You could also use the same licence on the following sub domains if they existed.
Please follow these instructions to get it work.
1 Install the free version of the software in your sub domain.
2 Copy the main domain "licence.lic" file to your sub domain.
3 Using a text editor, open the main domain "licence.lic" file and view your current main domain product key. Highlight the 60 character product key and do Ctrl-C to copy it to your clipboard.
4 Using a server tool like PHPMyAdmin or Adminer, log into your sub domain database and navigate to the "prodKey" column in the "settings" table of your sub domain setup. It will look something like this:
5 Update the sub domain key with your main domain product key (the one you copied in stage 3) and save in the database.
Done! Any problems, please post on the community forum.