Log in to your ThemeForest account and click on Downloads.
Locate your purchased product and click on the ‘Download’ button. In the dropdown menu that appears, click on ‘License Certificate’. You will be asked to ‘Open’ or ‘Save’ the certificate. Click on ‘Open’ and the Certificate will open in a new window.
Locate your ‘Item Purchase Code’ as indicated below. This is the code you will require to register at the UDTHEMES support forum.
Here is an animation of the whole process: