Set Default template for SSMS New Query button by editing SQLFile.sql

1. Go to SQL install path, for me it's "C:\Program Files (x86)\Microsoft SQL Server\140\Tools\Binn\ManagementStudio\SqlWorkbenchProjectItems\Sql"
2. Edit the SQLFile.sql in Admin mode
3. Type in whatever you want for default
4. Save and try in SSMS [New Query] button

Source
https://social.msdn.microsoft.com/Forums/sqlserver/en-US/b56b9ed6-1786-4ad6-87c9-7e4e81068672/how-to-set-the-default-query-for-the-ssms-new-query-button?forum=transactsql