When I create Marketo forms for external (non-Marketo) pages, I do the following which should solve your problem:
1) Create the form in Design Studio
2) Create a Marketo landing page in Design Studio and then Insert the form you just created.
3) You will then be prompted to select the Form, select "External URL" as the Follow-up Type, and then paste the URL of the follow-up page in the box.
4) However, these should be the same steps in the linked article above.
Hope this helps. Good luck.