Method 1:  In the H9 Control app:

• Pedal -> General Settings -> Use Expression Pedal, Aux Switch, or Both = Aux Switch, or Both.
• Pedal -> Aux Switch Mappings -> Set parameter 1
• Foot Switch 1 (Tip) [or whatever; just not one of the (… Momentary …) selections].
• Parameter Range ->  Start = 0  End = 40.

Method 2:  On the H9 itself:

• Hold Big Knob + right switch/Tap.  Dial to Aux SW.  Click Big Knob.  Dial to KBO: —
• Click right switch/Tap.  Dial to TIP [not TIP. with a trailing period.  That’s “Momentary“].
• Click left switch/Active.  Click left switch/Active once again.
• 0 will be the MINVAL.  Click left switch/Active a third time.
• With the Big Knob, dial 100 down to 40, as a MAXVAL.
• Hold Big Knob+right switch/Tap to exit the system mode.

Either method will control the MIX parameter (or similar), at those range limits.  Keep in mind that KB0 / Parameter 1 may be a completely different parameter, or scaled differently; depending on the algorithm.