Hi Cass
the reason you can't work out how to do it , is because I don't think you can.
Page 26 Manual
Quote
SELECTING A SINGLE TOUCH SETTING
(STS)
A Single Touch Setting (STS) contains Realtime (Keyboard)
tracks’ settings. Four STS are included in each
Style, and can be recalled by pressing the SINGLE
TOUCH SETTING (F‐1–F‐4) buttons under the display.
Note: You can select a STS only when you are in the main
page of the Style Play or Backing Sequence operating mode.

It's actually a style play function. It can be used in Backing Tracks

Quote
Backing Sequence
4 Realtime tracks, 8 Style tracks, 4 Pads.

There might be some sort of work around? but sorry, as I don't own one, I can't really test it out.