How to add fxs while mixing in flstudio

flstudio is a DAW (digital audio workstation), it is meant for recording studio use and music production only, unlike Ableton or Bitwig, it does not have a performance mode to quantize mix and effect mix.
So in a sense it is no DJ software to make mixtapes on and does not have things like beatgrid (well it does but not like in a DJ software, it has a totally different purpose here).

So I am sorry you are trying to use a software for something it is not meant to be used for.