I went looking for this walk through with a Squarespace site in mind. I thought it was still relevant with anyone wanting to do this with just CSS. The main change I had to make was when I installed it on the Squarespace site I was working on, I had to make the
display:inline-block for the button to be a better fit.
The link below will take you to their article. It was helpful for me and I hope it helps you with your project. Big shout out thesepapersites.com for the post even if it was back in 2016...
PS: I was contemplating whether or not to share this to LinkedIn and I hadn't looked at the sharing options to LinkedIn for a bit. Turns out, you can't share from Squarespace to LinkedIn.com automatically. I'm guessing they are strict about their APIs.