This feature is activated when your site goes live.
You can access webmail at:
Be sure to use your domain name and add /webmail to the end.
Login with the full email address and the password that was set when that email address was created. If needed you can reset the password for an email address.
When you login at the top left you'll see Roundcube. Below that you'll see an option to open your inbox when you login. If you want Roundcube to open automatically when you login to webmail select that option then click the open button.