CSGO use Protobuf, so you have to use
Protobuf instead of bitbuffer.
about the usermessage, you can see at CSGO sdk, and the path will be here:
X:/alliedmodders/hl2sdk-csgo/public/game/shared/csgo/protobuf/cstrike15_usermessages.proto
edited:
also, you can run the CS:GO client side with dev mode, and then you can see whether the usermesssage is valid or not.