Make SQL Server SSDT Database Project Reference Different in Publish Profile

In a SSDT Database project, I know how to publish the Self-Database to different locations. I can change eg, the Customer database name in the Publish profile. Example In: - Dev Environment: will publish profile to CustomerDevDB - QA Environment: will publish profile to CustomerQADB - Prod Environment: will publish profile to CustomerProdDB (This is legacy system, I did not design it like this) Now in our Customer Database, I have stored procedures which point to a Product database reference. How do I change my Project reference to be variable for each Publish Profile? Dev Environment: will publish to CustomerDevDB and will point to ProductDev QA Environment: will publish to CustomerQADB and will point to ProductQA Prod Environment: will publish to CustomerProd and will point to ProductProd I am trying to test around with Project DB Reference, and SQL Command variables. * I can point the Database Reference to different databases names, however I cannot change the reference in the publish profile yet (which is what I am really looking for). If someone can print screen images of how to do this, it would be helpful. Thanks