Use enroll_fingerprint(1) to enroll a fingerprint for authentication.
To enroll your right index finger (7th) do the following:

$ enroll_fingerprint -f 7

Then you can edit login.conf to create a login class that uses
fingerprint authentication:

#
# The fingerprint login class allows the fingerprint and passwd
# authentication methods and checks your 7th (right index) finger.
#
fingerprint:\
        :auth=-fingerprint,passwd:\
        :x-fingerprint=7:\
        :tc=default:
