The most common way to do this is with triple backticks:
Some regular text
```
Preformatted text
```
While this isn't in the original spec (https://daringfireball.net/projects/markdown/syntax#precode) it's supported by GitHub, Stack Overflow, and most other places I find myself using a markdown flavor.This syntax even made its way into CommonMark (https://spec.commonmark.org/0.29/#fenced-code-blocks)
I've yet to see what I'd really like: preformatted text being surrounded by markers that make it easy to paste in, like so:
It would be cool if the extension could do that and then do all of that obnoxious formatting for me. Or heck, it could be a full up WYSIWYG editor.