As I understand your request:
User visits domain.tld, which you do not control, they fill-in and submit a form on that domain. You want the user to be redirected to somewhere else after they submit the form.
Ethically, you would need to control the initial website in which the form resides. Then, do as I mentioned above, append the redirect to the form action process.
If whoever does control that site uses code that references a system you do control, you may be able to inject your redirect, but it would need to be a customized solution.
Since you do not have control of the domain/form being used you cannot ... control it.
There are other options which may allow you the ability to redirect the user in this instance, but they are ethically lacking. If you were able to install an application/malware onto the user's system you could accomplish this. It does not have to be as nefarious as that though, as you could get the user to install a browser "plugin" or "addon" or what have you, which provides some type of functionality, but that also redirects when this form in question is submitted. There are other ways to accomplish this, but I am not sure how they will fall into public domain on this site ... er ... even though this site is called "black hat" world I am not sure who would be willing to post truly "black hat" techniques to accomplish your redirect.