We are going through Marketo implementation now and have come to a roadblock. Does anyone know how to use a Marketo form submit to create a case in Salesforce? We have the custom logic built in SF.
What do you mean by "the custom logic"? An Apex REST endpoint?
The problem -- a really big one -- with sending Marketo form data *directly* to SFDC via a webhook (i.e. not via the standard sync) is you will create duplicates for anyone not yet present in both systems. That is, the webhook will create the person in SFDC along with a case, then that person will be slated to sync to Marketo. And at the same time the form post will have created them in Marketo, ready to sync to SFDC.
You should explain the desired logic more fully.