I did this

curl -v http://127.0.0.1 --http2-prior-knowledge --header ":scheme:aaa"

• Trying 127.0.0.1:80...
• Connected to 127.0.0.1 (127.0.0.1) port 80 (#0)
• 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
• Failed sending HTTP request
• Connection #0 to host 127.0.0.1 left intact
  curl: (55) Failed sending HTTP request

I expected the following

I expected the :scheme header to be passed for the http2 request and get a server response.

curl/libcurl version

curl 7.77.0 (x86_64-apple-darwin21.0) libcurl/7.77.0 (SecureTransport) LibreSSL/2.8.3 zlib/1.2.11 nghttp2/1.42.0
Release-Date: 2021-05-26
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtsp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS GSS-API HSTS HTTP2 HTTPS-proxy IPv6 Kerberos Largefile libz MultiSSL NTLM NTLM_WB SPNEGO SSL UnixSocket

operating system

Mac OS X 12.1 Monterey.