There is not way to do this unless using web services. You cold have a webhook that you fire from Marketo and calls a webservice thta writes that information in SFDC. Whatever you do, you will not be able to write to SFDC custom objects from within Marketo.
Another solution would be to consider the guests as separate leads with a different program status in Marketo and campaign member status in SFDC. In this case, you could use the /save endpoint and a webhook to call it and pass the guests to a specific form. You will find some information here: https://nation.marketo.com/message/166901-re-using-qualtrics-webservice-to-call-a-marketo-form-and-pass-survey-data#comm… and here : Simplest way to associate a contact coming from CRM with the Marketo cookie value
Hi Benjamin, unfortunately, I don't have an answer for you, but I am wondering if you could elaborate on how you handle event registrations in general? I've been trying to find the best way to take registrations and sync them with Salesforce, but so far, it's fairly cumbersome. I'd love to know how you do it.
(Sorry for highjacking your thread!)
Hi Brad - We create event programs in Marketo and sync them with Salesforce Campaigns. On form submit a flow changes the event program/sfdc campaign member status to 'Registered'. Day of the event we use the Marketo iPad event app to check people in and their status is auto changed to 'Attended' in the event program/SFDC campaign. For large events we use CVENT, however I wouldn't recommend them yet as they are overhauling their Marketo integration.
The problem with this approach as stated above is the inability to effectively capture guest registrations.