What do you mean by 'shortcut' letter ?
creates a shortcut (alt+b) for the button which, it seems, doesn't work while the msgbox
Certain controls may contain an ampersand (&) to create a keyboard shortcut, which might be displayed in the control's text as an underlined character (depending on system settings). A user activates the shortcut by holding down the Alt key then typing the corresponding character. For buttons, checkboxes, and radio buttons, pressing the shortcut is the same as clicking the control.[...]
Perhaps someone knows a trick to enable this shortcut while a regular msgbox is displayed (although that's probably not the point of a msgbox) - if not, perhaps build your own pseudo-msgbox with another GUI or use a regular AHK-hotkey
(perhaps context-specific: only when your GUI is active) to jump via gosub
to the gButton
label, if you really need this functionality while a msgbox is displayed...