If you don’t assign it to a widget, your sustain controller will produce a CC#64 message which will be sent to your plugin if it is part of the MIDI in block connected to your plugin. When receiving a CC#64, a plugin will do what it is supposed to do with it : mostly a sustain effect, but not always, e.g. organ doesn’t usually have a sustain effect implemented.
Having said that, what you want to do is not clear to me.
- If you want to prevent the CC#64 produced by your MIDI controller to trigger a leslie effect, than block the CC#64 in the corresponding MIDI in block.
- If you want to benefit from a sustain effect on plugin having no sustain, you will find a ready to use sustain Scriptlet here:
[Gig] Sostenuto and Sustain - If you want to assign your sustain controller to a plugin parameter different from a sustain parameter, and still benefit from the CC#64 message, then use the “THRU” option as mentioned by @pianopaul
There are many possible options depending on what you want to do exactly…