The field 'Assigned To (Workflow Support)' on the Marketing Opportunity object was intended to be utilized with workflows to assist in automating updates to the 'Assigned On' lookup field and was originally included in our i360 package in 2015. However, the field is now deprecated and no longer functions.
As an alternative to using this field in a workflow to update the 'Assigned To' field, we recommend utilizing the process builder to update this field, as it cannot be done with a workflow.