ItemCategoryUserField::add

public function add(int $categoryId, string $fieldName)
{
	if(!$this->isSupported())
	{
		return null;
	}

	$this->assertValidCategory($categoryId);

	$data = [
		'ENTITY_TYPE_ID'  => $this->entityTypeId,
		'CATEGORY_ID'     => $categoryId,
		'USER_FIELD_NAME' => $fieldName,
	];

	/** @var AddResult|null $result */
	$result = null;
	try
	{
		$result = ItemCategoryUserFieldTable::add($data);
	}
	catch(\Exception $exception)
	{
		throw new AddException($this->entityTypeId, [$exception->getMessage()], 0, '', 0, $exception);
	}

	if(!$result->isSuccess())
	{
		throw new AddException($this->entityTypeId, $result->getErrorMessages());
	}

	return $result->getId();
}