Across the Office suite:

Typing <word><hyphenminus><hyphenminus><word><space> yields an em dash.

Typing <word><space><hyphenminus><hyphenminus><space><word><space> yields an en dash.

That this has been true for some 3 or 4 decades makes me doubt all the comments that em dashes are a "tell" of LLM authorship. On the other hand, I guess when we confine this possibility to web content, I can see how people haven't used Office for web authoring lately, and whatever they do use (like web-based content management systems) don't tend to have this feature.