SimonRWaters on "[Plugin: Security Headers] Add HSTS preload flag"

// HSTS
    if (is_ssl()){
      $time = esc_attr(get_option('security_headers_hsts_time'));
      $subdomain = esc_attr(get_option('security_headers_hsts_subdomains'));
      $preload = esc_attr(get_option('security_headers_hsts_preload'));
      if ( ctype_digit($time)  ) {
        $subdomain_output = $subdomain > 0 ? "; includeSubDomains" : "";
        $preload_output = $preload > 0 ? "; preload" : "";
        header("Strict-Transport-Security: max-age=$time $subdomain_output $preload_output");
      }
    }

https://wordpress.org/plugins/security-headers/

Added to git, attributed to "Jamie", if you let me know your last name I'll add it to the readme.txt too.