There are several ways the HiGHS library can be used within another C++ project.

Use HiGHS from another CMake Project

Make sure HiGHS is installed locally with the correct CMake flags:

cd HiGHS
mkdir build
cd build
cmake -DFAST_BUILD=ON -DCMAKE_INSTALL_PREFIX=/path_to_highs_install/ -DCMAKE_TARGETS=ON ..   
cmake --build . 
cmake --install .

This installs HiGHS in /path_to_highs_install/.

Suppose another C++ CMake project has executable code in some file main.cpp, which includes Highs.h. To use the HiGHS library, edit the CMakeLists.txt as follows:

project(LOAD_HIGHS LANGUAGES CXX)

set(HIGHS_DIR path_to_highs_install/lib/cmake/highs)

find_package(HIGHS REQUIRED)
find_package(Threads)

add_executable(main main.cpp)
target_link_libraries(main libhighs)

The line

set(HIGHS_DIR path_to_highs_install/lib/cmake/highs)

adds the HiGHS installation path to HIGHS_DIR. This is equivalent to building this project with

cmake -DHIGHS_DIR=path_to_highs_install/lib/cmake/highs ..

Compiling and linking without CMake

An executable defined in the file use_highs.cpp is linked with the HiGHS library as follows. Make sure HiGHS is installed in install_folder following the installation guide. Afterwards, compile and run with

g++ -o use_highs use_highs.cpp -I install_folder/include/ -L install_folder/lib/ -lhighs

LD_LIBRARY_PATH=install_folder/lib/ ./use_highs