code: public CMAWebhook addHeader(String key, String value) {
      if (this.headers == null) {
        this.headers = new ArrayList<CMAWebhookHeader>();
      }
  
      this.headers.add(new CMAWebhookHeader(key, value));
      return this;
    }
docs: Adds a custom http header to the call done by this webhook.
  
  @param key   HTTP header key to be used (aka 'X-My-Header-Name')
  @param value HTTP header value to be used (aka 'this-is-my-name')
  @return this webhook for chaining.