Not sure how to best collect data from my home page button

Hello! I'm new to Squarespace and not highly tech savvy. Our old site was done on wordpress with a web expert. I've rebuilt most of it on Squarespace and am pretty  appy with the results. However I am not sure how to best collect data from our buttons/forms and get it over to Mailchimp with tags for automation. 

We have a "Great started for free" button all over our main page. I originally was using the form button, but this has some issues. I couldn't resize it, and I had to make a new form each time I wanted the button on the page. So I am hoping I can use a regular button. But I'm not sure how to "link" that to a lightbox form (ideally) that will collect emails for our lead magnet / challenge. And then ultimately get that information into Mailchimp with a tag. It seems like this would be a common use case, but I'm stumped . . . 

Any other feedback or suggestions on the site are welcome too. I'm still working on the "members page" which I am connecting via Memberspace (also new for us).

Thank you in advance!!