[V1ceC1ty]

pawn Код:

#define PRESSED(%0) \
    (((newkeys & (%0)) == (%0)) && ((oldkeys & (%0)) != (%0)))


public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    if (PRESSED(KEY_HANDBRAKE))
    {
    new Float:x, Float:y, Float:z;
        GetVehicleVelocity(GetPlayerVehicleID(playerid), x, y, z);
        SetVehicleVelocity(GetPlayerVehicleID(playerid) ,x ,y ,z+0.3);
    }
    return 1;
}

Press space and it will do what you want, if you want to change it to a different key then go there https://sampwiki.blast.hk/wiki/GetPlayerKeys#Key_List