😝 OK, you've actually not clarified it. 😁 It is still unclear 😕 if you mean velocity or aftertouch (aka pressure)
Velocity is how quick/forceful you play a key. You seem to mean this, judging from your second sentence.
Aftertouch (aka Channel Pressure, or, rarer, Polyphonic Pressure)* is the force with which you HOLD the keys down (that's why it is called after-touch). If you mean velocity, then my guide will do what you ask. Have you tried it?
* Most cheaper MIDI controllers and synths don't have aftertouch, and only some top enders have polyphonic aftertouch (=every key with individual AT-sensor)
http://www.harmonycentral.com/t5/Keys-Synths-amp-Samplers/keyboards-with-Poly-Af tertouch/td-p/310574