KConfig
KConfigWidgets provides easy-to-use classes to create configuration dialogs, as well as a set of widgets which uses KConfig to store their settings.
Using the Module
Using a KDE module's C++ API requires linking against the module library. The preferred way of consuming the library is through CMake.
Building with CMake
Use the find_package() command to locate the needed module component in the KF6 package:
find_package(KF6 REQUIRED COMPONENTS ConfigWidgets)
target_link_libraries(mytarget PRIVATE KF6::ConfigWidgets)
For more details, see the Build with CMake overview.