If I’m understanding correctly, several ways to do this, set up as momentary switching.  For example, a Preset saved in Bypass, with Active (M) kicking it in.  Or a fully active Preset, with a HotSwitch (M) that only punches in the harmony.

This would all be found under the Perform pages for easy access on-pedal.  Most latching controls in the H9 Gen 2 have a complementary option for momentary triggering.