Hi I am using this script JS script

function land(ref, target)
{
lowtarget=target.toLowerCase();
if (lowtarget=="_self") {window.location=loc;}
else {if (lowtarget=="_top") {top.location=loc;}
else {if (lowtarget=="_blank") {window.open(loc);}
else {if (lowtarget=="_parent") {parent.location=loc;}
else {parent.frames[target].location=loc;};
}}}
}
function jump(menu)
{
ref=menu.choice.options[menu.choice.selectedIndex].value;
splitc=ref.lastIndexOf("*");
target="";
if (splitc!=-1)
{loc=ref.substring(0,splitc);
target=ref.substring(splitc+1,1000);}
else {loc=ref; target="_self";};
if (ref != "") {land(loc,target);}
}

<form action="search" method="post">
<select name="choice" size="1" onchange="jump(this.form)" >
<option value="">List </option>
<option value="{$base_dir}1.html" >First</option>
<option value="{$base_dir}2.html">Second</option>
<option value="{$base_dir}3.html">Third</option>
</select><form action="search" method="post">
</form>

My question is: how can I make this when on click that it will open an iframe or a thick box?

Thank you for the help.