You have to have a full link, as you see when you hover over the module whether to edit it or to run it. To run the same module, as in the action attributed of a form, you would use "index.php?a=112&id=$moduleId". 112 is the action for running a module (see the manager/index.php file for all the possible actions).
That’s the whole purpose of MODx
Please do fold, spindle and mutilate - then share!