I tried both the FOLDER property as well as the CMAKE_FOLDER variable, neither worked. I have set USE_FOLDERS to ON.
set(USE_FOLDERS ON)
set(CMAKE_FOLDER "Temporary")
add_library(Example STATIC Example.c)
set_target_properties(Example PROPERTIES FOLDER "Temporary")
I manually tried adding a folder through Visual Studio as well, which added to the .sln (I used WinMerge to see the differences)…
	GlobalSection(SolutionProperties) = preSolution
		HideSolutionNode = FALSE
	EndGlobalSection
	GlobalSection(NestedProjects) = preSolution
		<.vcxproj guid> = <group guid>
	EndGlobalSection
CMake version is 3.30.4