On the pedal itself, follow the 6.4 Parameters process described there.  Perfectly illustrated in Fig. 6.25.  Instead of the MIDI parameters shown in the next section 6.4.1 Mapping, you’ll be dialing in an AUX switch (1 to 6).  Fig. 6.29 is where you’ll adjust the range from 0.0 & -6.0 dB.

It’s even easier using H90 Control, just mouse clicks & slider controls.  It’s also possible to add more versatility, by mapping the AUX switch to a HotSwitch function.

That is, other parameter changes can be mapped along with the Out Gain, as a ‘macro’.  Then all of it gets triggered – momentarily or latching – by the Aux switch, or onboard Performance switches.