Add a parameter under a group (created if needed).
        :type parameter: str or Parameter
        :type group_name: str

def add_parameter_to_group(self, parameter, group_name):
        """
        Add a parameter under a group (created if needed).
        :type parameter: str or Parameter
        :type group_name: str
        """
        groups = self.metadata \
            .setdefault("AWS::CloudFormation::Interface", {}) \
            .setdefault("ParameterGroups", [])

        if isinstance(parameter, BaseAWSObject):
            parameter = parameter.title

        # Check if group_name already exists
        existing_group = None
        for group in groups:
            if group["Label"]["default"] == group_name:
                existing_group = group
                break

        if existing_group is None:
            existing_group = {
                "Label": {"default": group_name},
                "Parameters": [],
            }
            groups.append(existing_group)

        existing_group["Parameters"].append(parameter)

        return group_name