First make sure you have the respective sent newsletter visible on the website (which is on the Newsletters page): in Communicate > Newsletters > section 'Sent Newsletters' > switch on the newsletter in the 'On website?' column to its right side.
Then go to Website > Pages > scroll down to the 'Special Pages' section > click on the orange link icon to the right side of your 'Newsletters' page, which takes you to the page on the website.
On the Newsletters page, look for the newsletter in question and click on the link 'open in new page' under the title of the newsletter. Once the newsletter is open in a new window, you can copy the link from there (from the navigation field), and then paste it on your social media platforms.
How does this work in version 3? I tried but couldn't make it happen