SSMS18 - Default File Location (Changed ssms.Application.pkgdef)
If using SSMS with a full install of SQL Server then there is a good answer on Stackoverflow.

If using SSMS as a standalone then the following should work


Locate C:\Program Files (x86)\Microsoft SQL Server Management Studio 18\Common7\IDE\Extensions\Application
Open ssms.application.pkgdef with a text editor(e.g. Notepad)
Change the value of DefaultProjectsLocation
"DefaultProjectsLocation"="C:\MYNEWPATH"
Save the file