BoxNote2Markdown provides the javascript to enable a bookmarklet, boxnote2markdown.js that, when run on a Box Notes page, converts the content of that page to markdown text.
Example bookmarklet code:
javascript:(function(){var s=document.createElement("script");s.src="https://communicatehealth.github.io/boxnote2markdown/boxnote2markdown.js";document.body.appendChild(s);})()