adding a mindbody login button to my squarespace menu

Test that page on your phone before you link it in the menu. Those widgets can take a second or two to load inside a code block, the page looks empty, and people tap away. A short line above it like "loading your account" helps more than you'd expect.

Squarespace nav items can only be plain links, so you can't drop widget code straight into the menu. What I'd do: get the login/register widget code from your Mindbody branded web tools, put it in a code block on a new page (something like /account), then add a navigation link pointing to that page. Code blocks need a paid Squarespace plan that allows custom code, so check that first.