code: public void addValueList(String key, String value) {
          Objects.requireNonNull(key, Required.KEY.toString());
  
          if (!valueMap.containsKey(key)) {
              List<String> values = new ArrayList<>();
              values.add(value);
              
              valueMap.put(key, values);
          } else {
              List<String> values = valueMap.get(key);
              values.add(value);
              
              valueMap.put(key, values);
          }
      }
docs: Adds an additional item to the value list
  @param key The name of the form element
  @param value The value to store