Creating window on Mouse action

Is there any Tutorial/Documentation on how to create window when clicking an object/npc, like a quest window?

I only found that one

But this doesnt help me.

Check this answer here with an example on how to create/show/hide a `GUI.Window` to get an idea.

The GUI Scripting Guide is something that you have to go through to get familiar with what kind of tools you have and it's not really hard to implement an OnMouseDown for an object to activate your window afterwards.

There are a lot of ways of implementing it and I can give you a simple example if you want.