FWIW, you can also get a temporary function by using the existing callbacks.
That’s what I’ve done for many years because I could dedicate only one button to speedbrake callbacks.
So I got this in my .key file :
AFBrakesOut 128 -2 -2 0 0x0 0
AFBrakesToggle 128 -2 -2 0x42 0x0 0
Which means that when you press the button # 128 the speedbrakes open, whereas they “automatically” close whenever you release the button. That way you can’t get confused in combat : the speedbrakes are open only if the button’s held.
As Airtex suggested, if you have enough buttons left, you can also choose to use that button as an “autoclose” button, and use 2 other buttons to open and close the speedbrakes with the “regular” callbacks.