code: public void addMergeField(String name, FieldType type) throws HelloSignException {
          if (!FieldType.checkbox.equals(type) && !FieldType.text.equals(type)) {
              throw new HelloSignException("Only 'text' or 'checkbox' types allowed for merge fields.");
          }
          mergeFields.put(name, type);
      }
docs: Add merge fields to the template draft. These are fields that your app
  can pre-populate whenever the *finished* template is used to send a
  signature request.
  
  @param name String name of the merge field that will be displayed to the
  user and used to key off the custom field when populating the
  value.
  @param type FieldType (currently only "text" and "checkbox" are allowed)
  @throws HelloSignException thrown if there is a problem adding the merge
  field.