vallidation part in javascript

Your question is not so clear. I guess you are entering a code in a textbox and want to show the name according to the entered code.

If yes then set AutoPostBack Property of the TextBox to True. And fire a TextChange Event. In the TextChange event write "C# code" to get the Name of the entered code in TextBox.
Try this:
HTML:

<asp:textbox id="txtPostCodefirst" runat="server" maxlength="6" autopostback="true" ontextchanged="txtPostCodefirst_TextChanged" xmlns:asp="#unknown" /> 

Code Behind:

protected void txtPostCodefirst_TextChanged(object sender, EventArgs e)
{
     //Write your code to get the name here..
}

--Amit