Normally, after you download and install the software from DVDFab website https://www.dvdfab.cn/download.htm, simply run the software, click the hamburger-like "Menu" button on the top right corner, click "Authorize", and input your account information associated with the product(s) to complete the authorization process.
To get detailed instructions, please refer to: https://www.dvdfab.cn/how-to-register.htm