Proof of concept to use a partial username or even another

CommunitiesLogin.vfp

<apex:page id="loginPage" controller="CommunitiesLoginController" title="{!$Label.site.site_login}" showHeader="false">
    <apex:form>
        <apex:pageMessages></apex:pageMessages>
        Username:
        <apex:inputText value="{!username}" />
        Password:
        <apex:inputSecret value="{!password}" />
        <apex:commandButton action="{!customLogin}"/>
    </apex:form>
</apex:page>

CommunitiesLoginController.apxc

global with sharing class CommunitiesLoginController {
    global String username {get; set;}
    global String password {get; set;}
    
    global CommunitiesLoginController () {}
    
    global PageReference customLogin() {
        String actualUsername = username;
        // here is where we would dynamically choose a field other than the username to retrieve the actual username
        if (actualUsername != null && !actualUsername.contains('@')) {
            actualUsername += '@7s-ray.dev';
        }
        return Site.login(actualUsername, password, '/');
    }
}