Sync-Time Custom Record Field for Composite Key

On 24.2 - I have a Product Record Type with a 1:n relationship to a Assignment Record Type. I need a custom record field on Product to show the "current assignee", which is one of the many Assignments, based on the product's current status and assignmentType. Since custom record fields do not support 1:n relationships, I thought to create a composite key following this recipe https://docs.appian.com/suite/help/24.2/custom-record-fields.html#create-a-composite-key. 

The problem is I cannot find where to mark the new custom record field as unique. What am I missing?