Authentication Flow without username

Hi, I would create an authentication flow that make me able to login asking user only his password.
Username is sent directly in the first request that Service Provider build versus Keycloak.
How can I do?

Can you help me?

1 -the user starts his navigation in the target application
2 - application redirect user to Keycloak to authenticate him
3 - user navigate automatiically to keycloak with a SAML request and an username parameter
4 - Keycloak show login page with only a password form
5 - Keycloak execute login operation using password inserted by form and username received implicitly.