[Gammix]

Its pretty straight using the callback:

pawn Code:

public OnMenuResponse(playerid, menuid, response, listitem)
{
    if(response == MENU_RESPONSE_SELECT)
    {
        if(listitem == 3)
        {
            //your code when the player selects listitem 3 (pressed the SPACE key)
        }
    }
    return 1;
}