How to copy-paste 'text' to clipboard using an add-on?

I’ve explored the documentation in MDN but couldn’t find a way to do this. Can anyone help.

You’d use the normal web API for this: https://developer.mozilla.org/en-US/docs/Web/API/Document/execCommand

Shows paste is not supported in Firefox 61

See the footnote in the compat table for Firefox.

Footnote has no mention about Firefox Quantum. It only has info about Firefox Gecko.

The footnote ends with:

image

And that links to a page explaining how to use execCommand in a WebExtension and getting access to the clipboard for reading (“paste”).

(P.S.: that linked page is also linked from the Browser Extensions overview page on MDN, sadly it’s not yet in the sidebar navigation so slightly harder to discover)