@property<EOL><INDENT>def substitution_tag(self):<DEDENT>
return self._substitution_tag<EOL>
A tag that will be replaced with the unsubscribe URL. for example:
        [unsubscribe_url]. If this parameter is used, it will override both the
        `text` and `html` parameters. The URL of the link will be placed at the
        substitution tag's location, with no additional formatting.

        :rtype: string