When setting up a custom object, you can only link it to a field that is visible to users or to the Marketo Id of either the person or the company. As for integrations it adds a level of complexity to maintain a repository of the unique Marketo Ids as mapped to the externalCompanyId it is a bit strange that this other unique field for the Company Object cannot be selected as a link field. Using a field that is visible to users as a link field introduced the possibility of user error in tasks like list uploads to mess up the data and lose the connection between your CO records and their Company record.
For use via the API it would be a great help to be able to use the externalCompanyId as link field.
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