If you want a share link in your browser, make a bookmarklet with the javascript
javascript:window.location="https://meta.discourse.org/new-topic?title="+window.location
(replace meta.discourse.org with whatever discourse instance you want to share to).
That will open discourse, open the composer, and insert the link into the title of the topic. You can even adapt that link to make it share to a specific category, or pre-fill some tags.
The same approach could easily be used on any site to add a “share to forum” button or similar.