Use the below code for listbox with SelectionMode attribute set to Single.
var istBoxElement = document.getElementById ( "ListBox1" );
var textBoxElement = document.getElementById ( "txtBox1" );
textBoxElement.value = elem.value;
and write this inside the onfocus event of the textbox element.
Sample Code
<script type="text/javascript">
window.onload = function() { BindEvents(); }
function BindEvents ()
{
var textBx = document.getElementById ( "txt1" );
textBx.onfocus = function () {
SetSel(this);
}
}
function SetSel (elem)
{
alert ( elem.id );
var elem = document.getElementById ( "ListBox1" );
document.getElementById ( "txt1" ).value = elem.value;
}
</script>
<div id="divMain">
<input type="text" id="txt1" />
<asp:ListBox ID="ListBox1" runat="server" SelectionMode="Multiple">
<asp:ListItem>1</asp:ListItem>
<asp:ListItem>2</asp:ListItem>
<asp:ListItem>3</asp:ListItem>
</asp:ListBox>
</div>