After creating and approving a Marketo Custom Object, the "Custom Objects" tab does not appear on person records, and the smart list filters for that custom object are not available.
When you create custom objects, you must provide "Link" fields to connect the custom object record to the correct parent record. Without at least one link field on the custom object, that tab will not display and the filters will not be available.
Follow the steps in Add Marketo Custom Object Link Fields to create Link Fields for either a one-to-many or a many-to-many structure for your custom object's association to the person records in your database.
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