FetchContent a directory but add a subdirectory

Code
15

OK, I had it working with the following code:

FetchContent_Declare(highs
    GIT_REPOSITORY https://github.com/ERGO-Code/HiGHS.git
    GIT_TAG "45a127b78060942721f75f46a04b274c2bb963d8"
    SOURCE_SUBDIR this-directory-does-not-exist
)
FetchContent_MakeAvailable(highs)
set(HIGHS_SOURCE_DIR "${highs_SOURCE_DIR}")
set(HIGHS_BINARY_DIR "${highs_BINARY_DIR}")
add_subdirectory("${highs_SOURCE_DIR}/src" "${highs_BINARY_DIR}/src" SYSTEM)