"Authentication Method"  is a method use to verify a user's identity when the client is trying to access/sign the tax form.

Navigate to gear wheel, and select "Settings" then press "Client Defaults"

Choose wither SMS or Birthdate, this will be applied for ALL the clients.

You may change the method via e-signature request, window before sending out the esign.

Related articles:

How to change e-signature signatory?

E-signature Authenticator (SMS or Birthdate)

Out To Sign: Using E-Signature  

Use tax agent as contact on all tax forms