def prepare_expand_argument(expand: str, default_fields: str) -> str:
    """
    The 'expand' command argument specifies which properties should be expanded.
    In this integration, several of the most significant characteristics are extended by default.
    Other attributes that users want to expand can still be provided.

    This method combines the default expand fields with the expand fields specified by the user.

    :type expand: ``str``
    :param expand: The expand argument passed by the user.

    :type default_fields: ``str``
    :param default_fields: The default fields.

    :return: expand argument value to send in request
    :rtype: ``str``
    """
    default_expand_fields = default_fields.split(",")
    custom_expand_fields = set(expand.split(","))
    expand_fields = ""

    for expand_field in custom_expand_fields:
        if expand_field.strip() not in default_expand_fields:
            expand_fields += f',{expand_field.strip()}'

    return default_fields + expand_fields