How do i make the 'Register' button to open the Membership registration & not default

Hi there,

I am trying to make one unified registration form,that will be accessible from the 'Register' button i have one the top right. Without needing the prospective member to register twice (once for the website, then again to membership plans) i just want to bring them straight in as a either free or paid member.
Many thanks for the help!

Best wishes,