How to add extra keys

observing a vertical shooter game PAT Shooter all the default player 1 keys (up, down, left right, z,x) were assigned I was trying to add more keys (for example 'A' for refill rockets).

btn(0),btn(1),btn(2),btn(3),btn(4),btn(5) -> all working as default player 1 keys but now I want to add
btn(6). Can anyone tell me how to do that