Action button on custom edit form

I have a list with 2 different Edit forms: 1 is the SP standard Edit form, and the other one is a form I customized.

I want to add a custom Action button to the custom form only. I have tried and could create a custom button, but it would apply to all Edit forms of the list (including the form where I do not want this button).

How can I add an action on 1 specific edit form only?

I just use Sharepoint Designer, but do not have advanced coding skill :(

Thanks for your help.