Bookmark it: your key is saved in the address bar after the #. Bookmark this page (Ctrl/Cmd+D) and one click shows your live code. The part after # is never sent to any server, so the key stays in your browser.
Security note: Your secret is processed entirely in-browser via the Web Crypto API and is only stored in the URL fragment, which browsers never transmit. Anyone with access to your bookmarks or browser sync can read it, so use this for development and testing; avoid saving live production secrets.