Set the query param in self.args based on the prefix and arg index
        and auto increment the arg_index

        :return: the string placeholder for the arg
        :rtype: str

def set_arg(self, value):
        """
        Set the query param in self.args based on the prefix and arg index
        and auto increment the arg_index

        :return: the string placeholder for the arg
        :rtype: str
        """
        named_arg = '{0}A{1}'.format(self.arg_prefix, self.arg_index)
        self.args[named_arg] = value
        self.arg_index += 1
        return named_arg