SQL Prompt sometimes works on some parts, e.g., it will give a list of columns for MATCHED but only for the TARGET table, never the source... It also suggest tables when coding a list of columns for an INSERT or it quits making any suggestions at all. This is a very hit-and-miss situation and very frustrating.
As the MERGE statement has been around since SQL Server 2008, it's very surprising/disappointing that, considering MERGE to be a STANDARD part of the DB Engine, SQL Prompt is this far out of date... FOUR YEARS AND COUNTING...!
Please "get current" :-)