Settings > Style Configurator > CMakeFile > COMMAND, add target_precompile_headers to User-defined keywords box ⇒ that will add highlighting for that keyword for you. (If you find other commands missing, you can add them to that same box, with spaces or newlines between.)
And if you really want Notepad++ to ship with that keyword in the Default keywords list in the future, you would have to put in an official feature request, and this forum is not the place for feature requests