Spawn mobs in a certain world

[CormanYT]

I wanted to spawn mobs in a certain world, or get them there.
First, I used this:

spawn zombie at {location} in world "WorldName"

That didn't work
Then, I used this:

spawn wither skeleton at world "WorldName"
teleport last spawned wither skeleton to {location}

That didn't work either. What does work? How do I spawn mobs in a specific world?

 

[Uzumaki]

if I'm not wrong that chunk must be loaded

 

[CormanYT]

if I'm not wrong that chunk must be loaded

Well, that doesn't answer my question. How do I spawn mobs in a certain area of a world?