Nothing to do here actually - 780 and 785 already allow to link by $w.

            <xsl:choose>
                <xsl:when test="$UseControlNumber = '1' and marc:subfield[@code='w']">
                    <a><xsl:attribute name="href">/cgi-bin/koha/opac-search.pl?q=Control-number:<xsl:call-template name="extractControlNumber"><xsl:with-param name="subfieldW" select="marc:subfield[@code='w']"/></xsl:call-template></xsl:attribute>
                        <xsl:value-of select="$f785"/>
                    </a>
                </xsl:when>
                <xsl:otherwise>
                    <a><xsl:attribute name="href">/cgi-bin/koha/opac-search.pl?q=ti,phr:<xsl:value-of select="str:encode-uri(translate($f785, '()', ''), true())"/></xsl:attribute>
                        <xsl:value-of select="$f785"/>
                    </a>
                </xsl:otherwise>
            </xsl:choose>