You can use the EditOperation.Merge method to merge features programmatically.
If you are looking to re-create the "Merge Construction Tool" available with Pro, you can create your own construction tool using the Pro SDK that is similar to what Pro does like this:
* Create a Custom Construction tool with options. Here is a wiki with sample code that walks you through this: https://github.com/Esri/arcgis-pro-sdk/wiki/ProGuide-Construction-Tools-with-Options This ProGuide demonstrates how to create a construction tool with options which allows users to input parameter values to the tool at run-time. So in this UI you can add logic to pick your features to merge, etc.
