@substitution_tag.setter<EOL><INDENT>def substitution_tag(self, value):<DEDENT>
self._substitution_tag = value<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.

        :param value: 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.
        :type value: string