function load_map()
{
if(key_x) // X pressed
{
level_load("second_map.wmb");
}
}

function load_map(STRING* filename)
{
    STRING* levelname = "#10";
    str_cpy(levelname, filename);
    str_cat(levelname, ".wmb");
    wait(1); //just for be sure the name is set
    error(levelname); //just for be sure the name is set
    level_load(levelname);
    wait(2);
}

function some_other_function()
{
    var keyget = 0; // the key_trigger, so the function isn´t called every frame!
    [...]
    while(1)
    {
        if(key_x == 1 && keyget == 0)
        {
            keyget = 1;
            load_map("second_map");
            wait_for(load_map);
        }
        else if(key_x == 0 && keyget == 1){keyget = 0;}
        wait(1);
    }
    [...]
}

function load_map()
{
  level_load("second_map.wmb");
}

on_x = load_map();