Hi Malik,
Hopefully you already found your answer, but just in case here's an alternate way to find the form_id value for a form. If I choose the "embed form" option in Marketo, I see the following code:
//<script src="//app-xx00.marketo.com/js/forms2/js/forms2.min.js"></script>
//<form id="mktoForm_1073"></form>
//<script>MktoForms2.loadForm("//app-ab03.marketo.com", "xxx-xxx-xxx", 1073);</script>
In my case, the id of the form I'm looking at is identified as 1073.
If you visit the form you'd like to use for POST capture and click "edit", you'll be bounced to an edit page with a URL that looks like this:
https://app-xx00.marketo.com/m#FOE1073
In this case, the #FOE1073 is the interesting part - your form_id value is visible here as the four-digit number at the end of the URL, which is again 1073.
Hope this helps!