code: public FoxHttpClientBuilder addFoxHttpPlaceholderEntry(String placeholder, String value) {
          foxHttpClient.getFoxHttpPlaceholderStrategy().addPlaceholder(placeholder, value);
          return this;
      }
docs: Add a FoxHttpPlaceholderEntry to the FoxHttpPlaceholderStrategy
  
  @param placeholder name of the placeholder (without escape char)
  @param value       value of the placeholder
  @return FoxHttpClientBuilder (this)