Is the Landing page a Marketo LP ?
- If so, just replace the form with a Marketo form.
- If not, you have 2 ways to do this,
- either submitting a Marketo form in the background (eadf Make a Marketo Form Submission in the background and all the comments
- Or simply replace your form with a Marketo embedded form. Read Embed a Form on Your Website - Marketo Docs - Product Documentation
What tool is your embedded form sending data to? What is the hosting service?
I agree 100% with Gregoire that replacing that form with a Marketo-hosted form is ideal. What is your business case for using a non-Marketo form in this instance?