You need what is called an "insert cable". This has a stereo (TRS) 1/4" jack plug on one end splitting out to a pair of mono (TS) 1/4" jack plugs on the other. Your PreSonus 22VSL has two channels that can take 1/4" jack inputs when switched to the "instrument" setting. You plug the stereo jack plug into the headphone out jack on the keyboard, the mono plug labelled "send" into the first channel of the interface and the second mono plug labelled "return" into the other channel.
You can now set up a stereo track in GB assigned to the pair of jack inputs, and you will be able to record your keyboard output in stereo.