Applies to Launchkey Mini Mk3

Open FL Studio and go to Options >  MIDI Settings

Select the Launchkey's first port as the Input and Output: 

Then set the Controller type to Novation Launchkey (Keyboard):

Finally, set "Send Master Sync" to on:

This will mean you can use the keys to play notes, use the Arp, Fixed chord and pads for note messages in Custom and Drum modes.  The dials can also be used to send your custom CCs.