Once you've created your passkey with FreeTaxUSA, the next time you sign in, you'll see a drop-down menu appear when you click to enter your username or password.
Selecting your passkey from that drop-down menu will prompt you to authenticate using your passkey (fingerprint, face ID, PIN, etc.).
A second or two after authenticating, you should be automatically logged into the app.
Why can't I sign in with my passkey?