Access MailChimp API 3.0 (GET)
You need to add the key via Basic Auth like and as far I am aware off, You can't query it from front-end, it must be on the back-end.
Find an example in NodeJS:
headers: { 'Authorization': 'Basic ' + new Buffer(`anything:${MailChimpKey}`).toString('base64');}
MailChimp not allowed to direct access with ajax. Once make Server WebRequest. It will surely work.