code: public function addFormField(IFormField $field, $isExtra = false)
      {
          $fieldId = $field->getId();
  
          if (empty($fieldId)) {
              throw new \LogicException('The access path of a form field must not be empty');
          }
  
          // setup the field and remember it
          $field->setParent($this);
          $this->children[$fieldId] = $field;
  
          // non-extra fields are tracked and automatically mapped from and to the subject
          if ($isExtra === false) {
              // setup the mapping context, remember it
              $mappingContext                  = new MappingContext($this, $field, $this->accessorChain);
              $this->mappingContexts[$fieldId] = $mappingContext;
          }
  
          return $field;
      }
docs: Add a field to the form
  
  @param IFormField $field   The field to add
  @param bool       $isExtra Non-extra fields are tracked and automatically mapped from and to the subject.
  
  @return IFormField