First you will need to set up either Face ID or a Fingerprint through your device. Use the links below if you need assistance in getting this set up.
Face ID: https://support.apple.com/en-us/HT208109
Now you just need to enable biometrics on the site. You can do so by clicking on the box next to "Enable Face ID" on the log in screen. You will now be able to use biometrics.