How do I add my fingerprint to other accounts

Your fingerprints are added ONLY to your PHONE.

Then, each online account which can use your phone’s TouchID as an authentication method is setup individually.

Usually it’s within the settings of the online account (or app) saying something like “Use Fingerprint” or “Use Biometrics”.

Sometimes the setting is even placed right on the “log-in” screen.

Unfortunately, developers often implement the steps of this setup differently; making the process inconsistent from site to site, or app to app.

See more info on Touch ID here:

Use Touch ID on iPhone and iPad - Apple Support