If you enter the username and password set for SYNC_USER1 in your custom sync server into the Email address and Password fields of the login dialog, it should be possible to sync with the custom sync server.
I assume that the reason a dedicated login dialog hasn’t been implemented is because there are probably very few users who actually use a custom sync server, so it’s likely a low priority. Anki desktop versions also reuse the login dialog intended for AnkiWeb.