One of the super cool functions on the Strymon BigSky is the Freeze/Infinite function, which allows you to achieve an infinite sustain sound. You can control that function on the BigSky itself by holding down the switch of the active preset on the BigSky. The function will disengage when you release the switch.
This function is also controllable via a MIDI Controller. In the BigSky MIDI implementation, sending a CC#70 message with Value 0 will change the parameter to Freeze, while a value of 1 will change the function to Infinite. This just changes the parameter, but does not engage the function. To do so, there is an implementation to mimic a switch press and release on the BigSky.
Sending a CC#97 message will mimic a Switch Press/Hold. When you want to mimic a switch press, send a CC#97 with value 127. The BigSky will take it as a switch is being pressed down. This will engage the Freeze/Infinite function. To mimic a switch release, you just need to send a CC#97 message with value 0. The BigSky will read it as the switch is being released.
There are several ways to program this on the Morningstar MIDI Controllers.
The first way would be to mimic the switch actions itself. The setting below will allow you to press Switch B to engage the Freeze/Infinite function, while releasing the same switch will stop the function. Do note that the Release action will have to be a "Release All" action, because if you hold down the switch, the Release action won't be engaged. Only the Long Press Release and Release All actions will.
A second option would be to utilise the toggle function, such that you just need to press the switch once to engage the Freeze/Infinite function, and then press the switch again to disengage it. The Preset Toggle function needs to be turned on, while the individual messages toggle position needs to be set to 1 and 2 respectively.
Of course, there are many other ways to do it, but this would be the most straightforward ways.
I hope that helps. Keep making awesome music!