cmake: set custom command for (parallel) build doesn't appear in the GUI ?!?

Setting custom command for build doesn't doesn't appear in

Projects -> Build Settings-> Build Steps -> Tool arguments:

or in

Projects -> Build Settings-> CMake -> Variables

  Executing the following cmake configuration, would make me expect to see in the GUI something anywhere.

add_library(myLib ${SRCS} ${HDRS})
add_custom_command(TARGET myLib COMMAND ${CMAKE_COMMAND} -j5})

Im not sure whats most appropriate approach to state explicitly the build arguments.