This works differently with Constellation than what you might be used to with UI-Kit or Theme Cosmos. You need to work the the Reference data type to implement a select pattern.
As a prerequisite, for the data page, you need to make sure the status is API.
I found multiple posts on this topic on the Pega forum, some with extensive details on the entirety of how to deal with CRUD, i.e. Managing external SoR through REST with CRUD actions. See if you can proceed with this!