HTTP2 protocol only works on systems with OpenSSL 1.0.2 or later. Hence, you can enable it on Debian 9, Ubuntu 16.04, RHEL, or CentOS, but not on Debian 8, Ubuntu 14.04.

To enable support for HTTP/2 on a front-end server, you configure the bind line in the configuration file's frontend section as an ssl endpoint, and alpn must announce h2, as follows:

frontend f_myapp
  bind :443 ssl crt /path/to/cert.crt alpn h2,http/1.1
  mode http