Hey Carol. Just go to your substack and grab the url to your home page. Ie mine is lindac.substack.com. Then on Medium, make a link the same way you usually do. Highlight a word and click the link icon and enter the url.
I think the confusing part is that your signup page on Substack doesn’t look the same to you as it looks to everyone else. If you log out of substack and look at your page, you’ll see what everyone else sees.
Hope that helps! :)