FYI. I moved your post into the Integrations category in Products and Support. You will probably get an answer there faster.
Also, these posts might help too:
And this content too:
Sharon, the safest and most scalable method is to use the Forms 2.0 API to inject a hidden Marketo form into the page and automatically populate and cross-post the form along with the Drupal form submission.
While other methods exist, they are rate-limited and if you have (or are) a skilled front-end developer, doing the client-side form post is the most straightforward and (since it appears to Mkto as if the lead her/himself posted) you have all the advantages of Marketo integration.
Thank you both. I'll show all this to the Drupal developers
Thanks Scott Wilder for your help. We ended up using Sanford's solution and it is working great. Thanks to you both!