Going to the right URL
To make sure people end up on, please do this:
In MailChimp,

1. audience

2. Manage audience

3. signup forms

4. form builder

5. forms and response emails -> confirmation thank you page

6. send subs to another URL

7. fill in something like

8. hit save


Get an URL like this:
NB: with post-json instead of post, you'll get JSON in return instead.

Setup e-mail address

1. From NameCheap, nameservers to CloudFlare

2. From CloudFlare, mx records to FastMail

3. Get e-mail, verify it in MailChimp