You can do Using
JavaScript
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script language="javascript" type="text/javascript">
function SetSelected(_this) {
document.getElementById('<%TextBox1.ClientID %>').value = _this.value;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ListBox ID="ListBox1" runat="server" Width="112px">
<asp:ListItem Text="" Value="">Test1</asp:ListItem>
<asp:ListItem Text="" Value="">Test2</asp:ListItem>
<asp:ListItem Text="" Value="">Test3</asp:ListItem>
<asp:ListItem Text="" Value="">Test4</asp:ListItem>
<asp:ListItem Text="" Value="">Test5</asp:ListItem>
</asp:ListBox>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</div>
</form>
</body>
</html>
Thanks,
Imdadhusen