Yes it is possible to set the left margin of the whole window.
But you should try to describe the exact result you want to achieve,
or ideally make a screenshot. Otherwise it can result in misunderstanding.

Notepad++ is a text editor not a word processor so it has no concept of a left-margin. Note however you can set margins when printing (see Settings (menu) -> Preferences… -> Print (box on left), but I don’t think this is what you are asking about.