How do I set the Constraint Name on a relationship in a macro?

I am able to get/set the 'BusinessName' of a relationship in a macro using rel.BusinessName,

However, there does not seem to be any property on Relationship that looks like it would hold Constraint Name, even though this a property within the UI

eg. 


    Dim rel As Relationship       
    For Each rel In mdl.Relationships

             rel.BusinessName = "xyz"      <--- this is fine

             rel.ConstraintName = "blah"    <--- this is not a valid property, and no other properties look relevant

        Next

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