I often add the URL in a comment. However, given the amount of requests for copies that I still get, despite almost all my articles being open access, this is apparently not the most effective way. Would be great if RG could add an option to add the URL when uploading the article in addition to the DOI, especially since the latter does not always effectively link to an article either.
Academia.edu has this function. It's true that such a feature would save a lot of time and effort. I have about 15 film critiques whose urls I'd like to link here. It would be far easier than finding the manuscripts on my computer and uploading them.