I wanted to create hotkeys for brush opacity and brush transparency for the ipad version of PSS,
and I wanted to share what worked for me.
1. I had to use a bluetooth keyboard, in my case the logitech ultrathin keyboard cover version.
2. Press and hold the return/enter key.
3. Tap the particular slider with your stylus or finger that you want to create a hotkey for like transparency or opacity.
(Note: if you tap the slider first and then press enter, this will not work. Nothing will happen.
You must press and hold the enter key first, then tap the slider!)
4. A Define Hotkey popup should appear.
5. Enter the "Define-" and "Define+" key or key combinations for each, or whatever options are available in the popup,
For example: since I'm right-handed, I use my left hand for the keyboard:
For transparency, I chose:
shift/s and shift/d
For opacity, I chose:
shift/w and shift/e
Make sure none of your choices are already used! You can check in the File>Define Hotkeys panel first.