First, you will need to make sure you consulted with the Kawai documentation for connecting the board to an external sequencer or computer. It would also need to be set to external MIDI clock.
Then you need to setup a New Instrument in Options/Preferences(OSX) > External Devices. And make sure to click the MIDI clock options in that New Instrument setup. Send MIDI clock, Use MIDI Clock Start, Send MIDI Timecode.
Here are some guides that show how to do that.
Please read these articles for setting up External Midi Devices Studio One 3.
Setting up to record with External Midi Device Studio One
Add 16 Instrument tracks each on MIDI channels 1 - 16. All armed for recording, with the MIDI outs of those tracks set to the New Instrument you create for the Kawai. If you have the Kawai setup for external clock, when you press play on Studio One it should start the sequencer on the Kawai.
Keep in mind this will only bring the MIDI notes over. Once the sequence has been finished. You will need to assign sounds to those tracks. Or you can let them keep triggering the MIDI from the keyboard. It can start to get pretty complicated without having the device in front of me, but that should get you started. Submit a support ticket if you have further questions.