I know what you what you want to do. You'll need to look at a 3rd party software as I stated above.
I change all my key bindings for my mouse and keyboard for gaming and regular use....but my high end gaming gear has that built into the software that each piece uses.
So on my standard keyboard....I remapped the F1 key to show the degree symbol.....it's 74°F right now. This applies to all standard keyboards....but, as I said previously, I don't know if that will work for on screen keyboard mode.
What type of keyboard is this?