How to measure the frequency response of the measurement system including the microphone and loudspeaker?

The problem with the loudspeaker is that it is usually not omni-directional. If you want to measure the response of your measurement system in one loudspeaker position you may try this procedure.

Do a typical impulse response measurement.

Make sure you plot the frequency response by selecting the plot type named Frequency Response - Spectrum. Do this from the list box in the Plot toolbar or from Plot->Plot Type/Routing/Test....

Place the microphone near the loudspeaker, 5 cm (2 inches) should be fine. Since it is so close to the loudspeaker it does not matter much if it the measurement is performed in a room.

Start a measurement.

Inspect the frequency response. It should not and will probably not be flat, but this measurement can be used to test the level and calibration.