This challenge reads:

The webpage initially looks like this:

The easiest solution is to use the curl command to construct a POST request with the required key/value pair.

curl -X POST <http://ctf.local:9000/json1/index.php> \\
-H "Content-Type: application/json" \\
-d '{"flag": "give_me_flag"}'

Alternate Route: Use Burp Suite!