Chaining authentication mechanisms with a single authentication module in OAM

I have created a custom page with two buttons. Each button will have a different authentication mechanism. I have also created a custom authentication scheme and a module for the same. The scheme has the challenge URL of the custom page. When we click on the application login URL, it redirects to the custom page. There is no issue till here.

Now the problem is that "how the authentication plugin of the module will be invoked on button click of custom page" ?

What should i write in form action ?

Note:- We are not taking any credentials on custom login page. We have just two buttons.