Currently there is no way to give any back-links from musing to external website. However, you can still mention the url in the post or answer on musing.
May be it is due to the fact that musing want people to post what their minds are and not what is written in other websites. Musing promotes original content, if the backlinks are disbled then probably people would start thinking of the questions and would help musing to grow as the platform where quality questions and answers are discussed.
Musing could be a good source of information in coming days besides google if the contributors make efforts to add the original content. Secondly, there is a possibility that they are figuring out how to manage the third party external backlinks which are allowed by their closest competitor Quora. A lot of people use quora to give backlinks to their websites though they are in the form of nofollow backlinks. Even steemit allows backlinks to be posted but the same is not considered in the google as dofollow unless your post have upvotes worth certain amount and your reputation is above certain level.
You can't give backlinks from musing.io to external websites because musing doesn't support backlinks. Even if you produce a back link in your answers to questions, it won't be clickable. Musing wants its users to provide original answers to questions without copying from the web and using backlinks to support what you copied.
Since you'll be providing something original, there'll be no need for backlinks. Though you can still share links if you want to but ensure that your answers are original and addresses the question asked before sharing such links