It sounds like you're an advanced user, so I'll give you an advanced solution.
I would capture the form submission using a Marketo form. Save the fields that need to go into the Custom Object into Marketo custom fields. Then use a webhook to call a script (PHP, .NET, Java, etc) and include all the Custom Object fields as parameters. The script will call the SFDC SOAP API to create the custom object.
You may want to include the SFDC ID in the webhook call (if the lead/contact is already synced with SFDC). I believe that these are the tokens:
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{{lead.sfdc lead id}}
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{{lead.sfdc contact id}}
(I may have the capitalization wrong, so please verify)