There are multiple ways of sidechaining in FL.
You have the classic way of sidechaining the sound that will trigger the reduction, to the sound that will be affected, and applying a compressor with a sidechain input (yes, the Fruity Limiter). Just search for tutorials for it on Youtube, you'll literally found thousands of it.
Or you can sidechain using the Fruity Peak Controller (look that up).
Or you can mimic sidechained pumping by using a volume envelope plugin (for example Gross Beat, or different thirdparty plugins).