In the Control Assign for the organ Part set VOLUME to respond to Aftertouch.
Select the Part containing the organ
Press [EDIT]
Touch “Part Settings” > “General” highlight VOLUME and set it to 0
Press the [CONTROL ASSIGN] button — this will allow you select a controller to control the organ Volume
A screen will appear telling you to activate the controller you wish to assign, apply Aftertouch to the Key. Now the organ will only sound when you apply Aftertouch.

You will be taken to the organ Part’s “Control Assign” screen, where Part “Volume” is the Destination, and “Aftertouch” is the Source

Thank You Bad Mister

Mr. Bad Mister when I set the aftertouch I can’t get my sustain pedal isn’t working. It work when I’m not using aftertouch. Also how do I set the sensitivity, how hard or soft the key is press to activate aftertouch.

There's no control over how hard you have to press to activate aftertouch. The throw of the key that does not activate aftertouch is a mechanical property of each key and should be "the same" (within reasonable tolerance) for all keys. All you can adjust is the region between the threshold of aftertouch activation and the "bottom out" of aftertouch where aftertouch is fully engaged at its maximum value. You can adjust the curve ratios/properties to adjust this region past the engagement threshold - which is the response to aftertouch once engaged.

I'm not sure I understand how aftertouch would change sustain. While you're engaging aftertouch your key is pressed - so it would sustain without the sustain pedal engaged. But the fact that aftertouch is engaged would not make notes that are not pressed continue to sustain. And lifting off the keys with the sustain pedal engaged should still allow the note to continue to sustain if you have not defeated the sustain pedal for that PART.

If something strange is happening with sustain - I would check the sustain pedal receive switch. Reference: https://www.yamahasynth.com/ask-a-question/yamaha-montage-how-to-switch-on-off-sustain-pedal-on-a-voice-within-a-multi-voice-performace

You may also want to check that the destination of aftertouch are not AEG (Amplitude Envelope Generator) parameters. Which could artificially keep notes from sustaining once aftertouch is engaged.

Mr. Bad Mister when I set the aftertouch I can’t get my sustain pedal isn’t working. It work when I’m not using aftertouch. Also how do I set the sensitivity, how hard or soft the key is press to activate aftertouch.

Aftertouch, is its own category of MIDI message and will not interfere with the Sustain pedal unless you have instructed it to also change the Amplitude Envelope. The Sustain pedal (cc64) will hold the envelope at a specific point, if Aftertouch is also programmed to alter the envelope it will do what it is instructed to do when applied.

Aftertouch can be applied when the key reaches the initial bottom of its travel. By adding pressure to a key at the bottom of its travel, you are pressing down on a sensor that sends values 0-127 in response to that pressure.

Similar to how the MW sends 0-127, or the Foot Controller sends 0-127, and so on. You make adjustments to what the values are assigned to do. For example, when the MW is set to flip the Rotary Speaker Speed from Slow (Chorale) to Fast (Tremolo) we know all values 0-63 would be Slow, and all values 64-127 would be Fast. If you use a ‘Standard’ Curve (linear) the MW will flip the speed when you physically reach halfway...

Using the Control Assign parameters for Curve, Ratio etc., you can shape the response so that distance you have to move the controller to send a value of 64 can be made closer or farther away from the physical halfway point.

You can ‘shape’ the response of the parameter assigned to the controller. If you have Aftertouch set to increase output level, you can shape the response so that the climb in volume happens immediately, or only after you apply a very significant amount of key pressure. The Control Set is all about defining the application - it’s Depth. It does not have to be linear!

Extra Credit: Adjusting to Velocity and Aftertouch
How hard one plays the keys has a lot to do with how useful Aftertouch can be for the player. If you are heavy-handed, you may sent Aftertouch simply by playing as you normally do. Aftertouch can be applied to Note-Ons with any Velocity. A note struck soft, with a Velocity of 40, can generate Aftertouch by adding pressure to the note once it reaches the initial bottom. A note struck hard, with a Velocity of 127, can also generate Aftertouch. But here, at the hard strike end of the scale, you want to be careful to NOT apply any additional pressure when the key reaches bottom... least you start sending Aftertouch, unintentionally. Unintentional Aftertouch can be trouble for your MIDI stream (it is like high cholesterol) it will eventually clog the system.

This is often misunderstood, but if you wind up with too much Aftertouch on your tracks (as a rule), first, please recognize that this does not happen to everyone, and second, this is an indication that you should adjust your instrument’s Velocity Curve. In other words you are reaching 127 too easily. Turn your sound system up, ...you will naturally play softer. Make these adjustments until you can play firmly without sending any unintentional Aftertouch.

If you play on a synth action, it is a good exercise to do - figure out how to adjust your effort and output, so that the data you are creating makes sense. If you look at your data and you have 127s everywhere, then you need to make some adjustments.
If you play on an 88 piano weighted action, remember the following:
127 is (or should be) the absolute hardest you are ever going to hit the keys. It literally makes no sense exceeding 127 in a modern electronic keyboard - it has no more to give you, period.

I like to think about it as ‘just short of abuse’. If you had a classic acoustic piano in your living room and someone can over and started banging mercilessly on the keys to where you wanted to say something, that’s 127, or this is how you should think about it. The hardest you would ever hit your instrument with the intention of making a musical sound.

Why adjusting the Velocity Curve to the “Hard” setting helps is obvious... it takes more effort to get loud.
Conversely, this is also why turning up your overall sound system works, because you can hear yourself really well you find less need to increase your effort and exceed 127 velocity.