def METHOD_NAME(self, identifier):
    """
    Gets API key (with prefix if set).
    :param identifier: The identifier of apiKey.
    :return: The token for api key authentication.
    """
    if self.api_key.get(identifier) and self.api_key_prefix.get(identifier):
        return self.api_key_prefix[identifier] + ' ' + self.api_key[identifier]
    elif self.api_key.get(identifier):
        return self.api_key[identifier]