MVC implementation, sort of. How to link two forms through an interface?

Question

Delphi

I am using C++ Builder.
There are two forms: FrmAuth - authorization form, FrmMain - main form.
It is necessary that they exchange "messages" with each other through the controller.
I want the controller to process the event from FrmAuth (onClick) when trying to authorize and pass the data (login, password) to the model, which in turn will attempt to connect to the UDB. If everything is successful, then it will return a message about this to the controller, which should close the authorization form and open the main form.
Also on the main form there is a button "Change user". When changing the user, messages go from the main form through the controller to the authorization form.
I do not understand the logic of linking everything together.
Something like that!