$ curl -v 'https://elastic.faked.org/api/clusters/_connect' -X POST -H 'User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:120.0) Gecko/20100101 Firefox/120.0' -H 'Accept: application/json, text/plain, */*' -H 'Accept-Language: en-US,en;q=0.5' -H 'Accept-Encoding: gzip, deflate, br' -H 'Content-Type: application/json;charset=utf-8' -H 'Origin: https://elastic.faked.org' -H 'DNT: 1' -H 'Connection: keep-alive' -H 'Referer: https://elastic.faked.org/' -H 'Cookie: VouchCookie=<redacted>' -H 'Sec-Fetch-Dest: empty' -H 'Sec-Fetch-Mode: cors' -H 'Sec-Fetch-Site: same-origin' -H 'Pragma: no-cache' -H 'Cache-Control: no-cache' -H 'TE: trailers' --data-raw '{"ip":"172.17.0.1","port":"9200","username":"","password":"","use_ssl":false}'

Note: Unnecessary use of -X or --request, POST is already inferred.
*   Trying 2003:a:37f:ef4f::1:443...
* Connected to elastic.faked.org (2003:a:37f:ef4f::1) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*  CAfile: /etc/ssl/certs/ca-certificates.crt
*  CApath: /etc/ssl/certs
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
* TLSv1.3 (IN), TLS handshake, Certificate (11):
* TLSv1.3 (IN), TLS handshake, CERT verify (15):
* TLSv1.3 (IN), TLS handshake, Finished (20):
* TLSv1.3 (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384
* ALPN, server accepted to use h2
* Server certificate:
*  subject: CN=faked.org
*  start date: Oct 27 21:01:46 2023 GMT
*  expire date: Jan 25 21:01:45 2024 GMT
*  subjectAltName: host "elastic.faked.org" matched cert's "*.faked.org"
*  issuer: C=US; O=Let's Encrypt; CN=R3
*  SSL certificate verify ok.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x561b92d10620)
> POST /api/clusters/_connect HTTP/2
> Host: elastic.faked.org
> user-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:120.0) Gecko/20100101 Firefox/120.0
> accept: application/json, text/plain, */*
> accept-language: en-US,en;q=0.5
> accept-encoding: gzip, deflate, br
> content-type: application/json;charset=utf-8
> origin: https://elastic.faked.org
> dnt: 1
> connection: keep-alive
> referer: https://elastic.faked.org/
> cookie: VouchCookie=<redacted>
> sec-fetch-dest: empty
> sec-fetch-mode: cors
> sec-fetch-site: same-origin
> pragma: no-cache
> cache-control: no-cache
> te: trailers
> content-length: 77
>
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* old SSL session ID is stale, removing
* Connection state changed (MAX_CONCURRENT_STREAMS == 128)!
* We are completely uploaded and fine