whmcsZimbraSingle/clientarea.tpl

<table width="100%" cellspacing="0" cellpadding="0" class="frame">
  <tr>
    <td>
      <table width="100%" border="0" cellpadding="10" cellspacing="0" class="table table-striped table-framed">
        <tr>
          <td align='left'>Webmail</td>
          <td align="left"><a href="{$webmailurl}" target="_blank">{$webmailurl}</a></td>
        </tr>
        <tr>
          <td colspan=2>
            <button type="button" class="btn btn-primary">
              <a href="/whmcs/clientarea.php?action=productdetails&id={$serviceid}&dosinglesignon=1" target=_blank color="white">
                <i class="fas fa-sign-in fa-fw"></i> Login to Zimbra
              </a>
            </button>
          </td>
        </tr>
      </table>
    </td>
  </tr>
</table>