[keycloak-user] Realm resolution based on username (email address)

[Pedro Pedro]

Hi. 
   I'm working on a multi tenant project where usernames are actually their email addresses and the domain of the email serves as a tenant identifier.  
  Now in Keycloak I'll have different realms per tenant, but I want to have a single login page for all tenants and the actual realm that will do the authentication to be somehow resolved by the username (email address).  
  How do I go about doing that?   
  Best regards, Pedro.