Button in main menu

Hi, I’ve created a form to pass credentials of a logged user(username & ISO_639_1) to an external website and it’s working fine. Now I need to create a button with a special ID to target the form and such button must be included into the main menu. I’ve read posts about adding a button to wp menu but they are all related to CSS classes and style and that’s not my case: I don’t need a button appearence but a button functionality. Since custom links accept only straight URLs I don’t have a clue on what to do to create this. Any clue? Thanks.