Description of the Issue
Pursuant to the direction HERE and HERE, let's move this Preference setting:

to be on the SaveAs dialog itself, appearing in a similar manner as the Save Folder as Workspace checkbox when one does a File > Save Session... :

Exact text for this that should be shown for this on the SaveAs dialog is TBD. The existing text from the Preferences area is perhaps a bit too long to use.
Debug Information
Notepad++ v7.9.3 (32-bit)
Build time : Feb 14 2021 - 03:12:31
Path : C:.........\npp.portable\notepad++.exe
Admin mode : OFF
Local Conf mode : ON
OS Name : Windows 10 Enterprise (64-bit)
OS Version : 1909
OS Build : 18363.1316
Current ANSI codepage : 1252