I work mainly with an iPad so my terms will be tablet and Apple centric, though sometimes it is easier to use the desktop.

Yes, it is allowed but you should post the actual link plus as a quote to attribute the author and source correctly. In simplest terms, you copy the url address from your browser and paste it in a reply or new thread box here.
Sample with details:

Copy the entire URL for the article from your browser bar (press command -c) and then paste it here (command-v). It should be in a link form automatically. Keep both windows open (lowcarb forum and the article)
Then to copy all or part of the article, go back to source on your other browser window, copy what you want, then here, press the yellow square (looks like a page) quote appears, paste the article, press yellow page again, and that now article appears inside a quote box.

I usually hit go advanced, preview post, and test to make sure the link really does work, the quote is complete with author name and source, then post.