I opened the webpage from the challenge description: https://52.196.116.69/index.php which resulted in Chrome showing SSL cert error: NET::ERR_CERT_AUTHORITY_INVALID.
I checked the CN of the SSL cert which was: very-secret-area-for-ctf.orange.tw so I created the following entry in my host file:
Then I opened the https://very-secret-area-for-ctf.orange.tw/ URL, which gave me the flag: