Force paid subscription plan

I haven't subscribed it for Emerald so forgive me if i'm not familiar with Emerald workflow.

I have 3 plans. Example:

1 - free

2 - 20 €

3 - 40 €

When user press the free it gets directed to a form? Since register is not integrated with Emerald what does it show next?

I'm saying this because it would be nice to have a mechanism on Emerald that when the users press FREE subscription, the next form i assume it has a "finish subscription" button.

It would be great that the "finish subscription" button would have an event that could be configured on the backend so that it drives the user to a paid plan with discount.

Here's a resume:

User press FREE subscription, next page is a subscription form with information of the user (i think) and when the user press the button "send now" it popups up an alert:

"Why free? purchase plan 2 and we will offer X % discount on the plan."

With option "no thanks" and "yes i want plan 2 with discount".