The S3 series does not use an API code.
You need to log into M5Burner with your forum ID and password and then during the erase and burn process you will be asked to bind the Atom S3 to your account.
Once you agree and complete the burn process, load UIFlow2, log in (again using your forum username and password), Click on the device selector at the bottom of the screen and you will get a list of devices registered to your account.