If you’re using the app @ritchierich linked then you have to define what the code will use.
If you’re using this app (i.e. no programming only notifications and actions), then you need to find out what slot you’ve programmed into your lock and enter the name against that slot number. If you’ve cleared all codes from your lock and starting from scratch it should start from 1 and work it’s way up.
If you don’t know what slot you’ve programmed in your lock, when you unlock the door using your code, in the device events page on your ST mobile app it will tell you which slot number was used to unlock the door.