How can we authorize without specifying particular connection when Auth-0 has multiple connections

We have multiple connections configured for a native android and iOS application. We are using custom login form and hitting /oauth/token api end point to get token and trying to use resource owner password flow. Requirement is to not pass any connection and let Auth0 pick up the right connection(realm) and authorize. It seems like realm is mandatory field for authorization. Is there any configuration in auth-0 website to skip the connection check for login.
I was going through below link and found out that there is some way to let auth-0 select connection without specifying connection in case of auth-0 lock.

But we are using custom login UI and not using any Auth-0 sdk. How can this be achived?