As I was interested I did adapt an existing widget-extension we have with CopyToClipboard. It just uses 'navigator.clipboard.writeText(copyText.value);' like described in https://www.w3schools.com/howto/howto_js_copy_clipboard.asp
It is not "offical" yet, still in my fork. But if you want to give it a try: https://github.com/Death111/CopyToClipboardWidgetTWX in Actions tab you can select the latest pipeline (current) run to download the extension (need to be logged in in github).
It did work fine in my test instance and Chrome. What I've read about the navigator.clipboad function is that it only works on https sites or localhost
