Gets API key (with prefix if set).

        :param identifier: The identifier of apiKey.
        :return: The token for api key authentication.

def get_api_key_with_prefix(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]