Something isn't working with my MessageBox.

I also had to figure it out for a long time. The problem was that I was typing messages in Notepad, and it automatically chooses the wrong format. Anyway, take a look at this topic; it has everything written about MessageBox.

Is it possible to describe in as much detail as possible how to set a task?

Do you mean, including the name and description of the task? Or did I misunderstand? Because the scripting part is exhausted by the SetObjectiveState function. You can also sometimes use SetObjectiveProgress.

How to configure DefeatHero: description, launch, and so that a video plays and the mission ends upon completion?

Here's how (+screenshot):

function P0_end()
  StartDialogScene("bla-bla");
  Win(1);
end;

function P0_start()
  SetObjectiveState("P0",OBJECTIVE_ACTIVE);
  Trigger(OBJECTIVE_STATE_CHANGE_TRIGGER,"P0","P0_end");
end;

Trigger(Trigger Type, parameters,"P0_start");

It's interesting how "Summon Ilm-Hijdra" from Lords works. Is it moving objects?

Yes, it seems to be a scripted spell that moves an object. Although I didn't delve into the script there.:)

And how do you take troops away from a hero?

RemoveHeroCreatures(heroname, creatureID, quantity);

I advise you to check the source mentioned above more often. It's a really great thing – I don't understand why people ignore it ;)