Get value of TextBox set using JavaScript in code behind (server side) in ASP.Net

Use Request.Form collection to get the value.

Refer below code.

HTML

<asp:TextBox runat="server" ID="txtgrncode" />
<asp:Button Text="Add" runat="server" OnClick="btnAdd_Click" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    window.onload = function () {
        GetAutoGRNCode(1);
    }
    function GetAutoGRNCode(SelectedGRNType) {
        $.ajax({
            type: "POST",
            url: 'Default.aspx/GetAutoGRN_Code',
            data: '{selectedGrnType: ' + SelectedGRNType + '}',
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function (response) {
                //alert(response.d);
                $("[id*=txtgrncode]").val(response.d);

            },
            failure: function (response) {
                alert(response.d);
            }
        });
    }
</script>

Code

protected void btnAdd_Click(object sender, EventArgs e)
{
    string GRNCode = Request.Form[txtgrncode.UniqueID].ToString();
}

[System.Web.Services.WebMethod]
public static string GetAutoGRN_Code(string selectedGrnType)
{
    string GRNCode = "1234";

    return GRNCode;
}

done using 

HttpContext.Current.Request.Form[txtgrncode.UniqueID].ToString();