Hi Gomathisri,
I have created sample that full-fill your requirement.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript">
function SelectedTextValue(ele) {
if (ele.selectedIndex > 0) {
var selectedText = ele.options[ele.selectedIndex].innerHTML;
var selectedValue = ele.value;
document.getElementById("txtContent").value = selectedText;
}
else {
document.getElementById("txtContent").value = "";
}
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div align="center">
<select name="ddlContent" id="ddlContent" onchange="SelectedTextValue(this)">
<option selected="selected" value="">Select</option>
<option value="1">aspsnippets.com</option>
<option value="2">aspforums.net</option>
</select>
<input name="txtContent" type="text" id="txtContent" />
</div>
</form>
</body>
</html>
Demo
You can change the html control with below asp controls aswell.
<div align="center">
<asp:DropDownList ID="ddlContent" runat="server" onchange="SelectedTextValue(this)">
<asp:ListItem Text="Select" Value="" Selected="True" />
<asp:ListItem Text="aspsnippets.com" Value="1" />
<asp:ListItem Text="aspforums.net" Value="2" />
</asp:DropDownList>
<asp:TextBox runat="server" ID="txtContent" />
</div>