Cant setup keyboard on Snes mini

[gbasource]

I'm trying to use a USB keyboard for playing spectrum games on the snes mini. The keyboard is only working in the retroarch menus and when I launch a game, no keys I press seem to do anything in the games. If I press start and select on my inserted wii classic controller pro, a keyboard overlay appears behind the retroarch menu. Keyboard doesn't even appear in the device list and when I try mapping the keys there only seems to be the the z, x, a, s, enter and shift keys that's ever in the list. Is it even possible to use a keyboard to play spectrum games on the snes mini?