Hey Sean,
It would look like this:
1. Users Fills Out Form
2. When the Submit button is pressed, during the onValidate or onSubmit callback, send an AJAX call to your webhook.
3. On AJAX success(or Failure), use the addHiddenFields() and vals() methods to create fields and values from the AJAX response which are posted to Marketo with the form submission.
4. Form is submitted, and then the followup is selected based on your rules for the form in Form Settings in Marketo.