Ok, so a note off message looks like this:
8c nn 00
c is the MIDI channel
nn is the note nubmer
00 is the release velocity. I put 00 here, because veloicty probably doesn't matter. (Hopefully it doesn't?)
Now, the unfortunate thing about Sysex is that the numbers are specified in hexadecimal, which is a very nerdy thing that end users really shouldn't be subjected to. But, it's been the standard for sysex for decades.
So, let's start with the first number: 8c. "c" needs to be replaced with your MIDI channel number. 0 = channel 1, 1 = channel 2, 2 = channel 3, etc. until you get to channel 11. A = channel 11, B = channel 12, etc.
nn is the note number, translated to hexadecimal. 58 in hexadecimal is "3A". You can use a site like this to do the translation for you:
https://www.rapidtables.com/convert/number/decimal-to-hex.html
So, if you want to send a note off for note 58 on channel 1, it would be:
80 3A 00
Go to the Sysex tab, select one of the Sysex messages in the left column, and put those numbers in the edit field on the right. Let's say you edited sysex #0. You could name it "Note off 58" or something like that.
On your button, leave the existing action and add a Sysex action: Sysex off:0 on:None. This will cause it to send the note off message when the button is turned off (or released in the case of a momentary button)
This workaround should get you going. I apologize for this being so complicated, note off has never been an issue before. I will look into adding an option to select whether notes send note on velocity 0 or note offs.