You can set the email with the login_hint parameter like this:

[email protected]

https://keycloak.mywebsite.com/auth/realms/myrealm/protocol/openid-connect/auth?client_id=myclient1&redirect_uri=https%3A%2F%2Fwww.mywebsite.com&login_hint=foo.bar

Here is a link to the documentation: https://www.keycloak.org/docs/latest/server_admin/