3 years and still this hasn't been implemented?
sigh...
Frankly, I find Marketo's support for custom objects to be lacking. You can trigger on adding a custom object... but that's where it ends.
Sigh. Sigh.
You can now Trigger Off Custom Object Changes - Marketo Docs - Product Docs
The need is to trigger off Salesforce custom objects, not Marketo custom objects
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