Custom object not showing under link object drop down
Hi guys,
I'm trying to Create a Link Field for a Many-to-Many Structure and have some issues.
Custom Object 1: Links to Lead ID and has a Product ID which is supposed to serve as link field for custom object 2. This custom object 1 is already approved.
Custom Object 2: This will link to Custom Object 1 using Product ID, but I'm not able to see custom object 1 in the link object drop down. Only options I can see are Person and Company.
Attached is the snapshot:

Am I missing something here? Or this is something that needs to be enabled from back-end by requesting Marketo support team?
Really appreciate any help on this as we are stuck atm and can't progress without this.