You can simply interact with chrome devtools to set virtual authenticator.

Karate has this capability DevTools Protocol Tips

Following is a simple feature file for webatuhn.io using virtual authenticator.

Feature: Authentication success flow for FIDO.

  Background:
    * configure driver = { type: 'chrome', showDriverLog: true }

  Scenario:  Verify authentication is successful
    Given driver 'https://webauthn.io/'
    *  driver.send({ method: 'WebAuthn.enable' })
    * driver.send({ method: 'WebAuthn.addVirtualAuthenticator', params: { options: {"protocol": 'ctap2', "transport": 'internal', "hasUserVerification": true,  "isUserVerified": true,  "hasResidentKey": true  }}})

    And input('#input-email', 'dummy')
    And click('#register-button')
    // wait here for a second or two
    And click('#login-button')

This approach might work only for Chrome, I have not explored it further. Just wanted to add this if anyone needed.