Change Link of Register Button in login form

By “register button” do you mean the link?

You can filter the link to the registration page in the login form using either the wpmem_reg_link filter to filter just the link URL itself of the wpmem_reg_link_str to filter the entire string of HTML that makes up the link (so you could set your own URL in the tag and filter the text of the link at the same time).

Docs for both:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_reg_link/
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_reg_link_str/

There is a list of documented filters here:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/