As Florent mentioned You can't use inbuilt form capability to post data third part systems. To solve this problem we have two solutions:
1. Host your form in website/third party system and use AC api to pre polutale the data in the form then you can specifiy ant post action in JS to that page which will behave like a normal HTML form.
2. In adobe campaign use a JS script as next action to the form and In JS activity you will have accsses to all in context data from the from then to write a code to make a HTTP post request to your targeted system.