code: func formatUserAgent(customUserAgent string) string {
  	if customUserAgent == "" {
  		return defaultUserAgent
  	}
  
  	return fmt.Sprintf("%s %s", defaultUserAgent, customUserAgent)
  }
docs: // formatUserAgent builds the final user agent to use for HTTP requests.
  //
  // If no custom user agent is provided, the default user agent is used.
  //
  //     dnsimple-go/1.0
  //
  // If a custom user agent is provided, the final user agent is the combination of the custom user agent
  // prepended by the default user agent.
  //
  //     dnsimple-go/1.0 customAgentFlag
  //