In Sounfont interpreters (soundfont player, virtual sound. synth, etc), this setting must exist by default.
But you can override:
In the Instrument section, select an instrument.
Make sure you click anywhere in the global section.
Click on the toolbox icon at the top and choose the command "Override a default modulator".
The option numbered "Default Mod. #2" is for this setting. // Note On velocity, \ , -2400, No op, Filter cutoff (Hz)
Select it and click the "Override" button.
You will see that it has been added to the Modulator section below and it says "Modulator #n overriding default mod".
Note: If added properly, a gray square should be formed around the cell of the "Filter Cutoff (Hz)" section in the global section.
If you leave the "-2400" value here as it is, your interpreter (the player / synthesizer where you loaded your sountfont) will fit it as standard. Thus: As the velocity decreases, the filter value increases.
If you think this value has little effect, you can also increase this value (try -4800 (for x2), -6000 (for x3), etc).
If you want to see how it works, try it by setting a value of -12000 (But remember, this is extreme, not useful, it's just to test how it works).
On the Polyphone's virtual keyboard, you can try how the value you set sounds at which velocity value. When you move your mouse up and down over a piano key, the Velociy values appear in the lower right part of the piano window.
(This Window is adjustable. You can make it bigger or smaller by pulling the edges).