How do you indicate an interjection with semicolons?

The em is more than a synonym for semicolon.

...someone can correct me if I'm wrong, but I don't think you're using the semicolon correctly; it sounds wrong to me, at least. I think you'd need to either take out the word "but" (which would cause the sentence to no longer make sense) or, well, change the semicolon to an em dash.

For sure, that's like the exact opposite of why to use it: "A semicolon is used to join two closely related independent clauses (complete sentences) that are not connected by a coordinating conjunction."

Which hyphen is that?