you can modify the exsisting captive portal by using following form in the index.html.
<br><br>
<b>CLICK ON THE BUTTON BELOW TO LOGIN AS GUEST.</b></font></div>
<div align="center">
<br><br><br>
<form name="form1" method="post" action="/auth/index.html/u">
<span class="bodytext">
<input type="hidden" id="email" name="email" type="text" value="user@company.com" class="text" accesskey="e" />
<input type="hidden" name="cmd" value="authenticate" />
<input type="submit" name="Login" value="Guest Login" class="button" />
</span>
</form>