call the input textbox value from the asp.net code behind

make the input textbox to

runat="server"

Try this one to access your HTML controls on code behind file

C#

<input type="text" id="txtId" runat="server" />

To fire onkeypress event you can call your javascript function from codebehind file.
you can refer this

This is the AsP.Net form (eg : Registraion.aspx)

<form id="form1" runat="server">
    <fieldset class="register">
             <asp:Label ID="AccountNameLabel" runat="server"                AssociatedControlID="AccountNameText">Name:</asp:Label>
             <asp:TextBox ID="AccountNameText" runat="server"></asp:TextBox>

        <asp:Button ID="CreateUserButton" runat="server" CommandName="MoveNext"
                                Text="Proceed with Registration" CssClass="button fright"
                                onclick="CreateUserButton_Click" />
    </fieldset>
</form>

The above code enables the TextBox control to be accessed from the Code behind file.
you can simply achieve this by(from - Registraion.aspx.cs):

string strName = AccountNameText.Text;

Could you Please refer the following links?
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.textbox.aspx[^]
http://www.w3schools.com/aspnet/aspnet_textbox.asp[^]

YOU ALWAYS CAN LEARN SOMETHING ONLY BY DOING RESEARCH BY YOURSELF RATHER ASKING THE SOLUTION FROM OTHERS. The question you asked here is very basic. If you have any questions first you go to goolge and search. Good luck.