[Konstantinos]

https://sampwiki.blast.hk/wiki/Keys

KEY_CROUCH is H/Caps Lock for vehicles. Also you need to add + 0.3 from the z height.

pawn Код:

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