How to programatically put telerik rad grid in “add new” mode on page load

Please try with the below code snippet.

ASPX

<telerik:RadGrid ID="RadGrid1" runat="server" AutoGenerateColumns="false" OnNeedDataSource="RadGrid1_NeedDataSource"
    AllowFilteringByColumn="true" AllowPaging="true" OnItemCommand="RadGrid1_ItemCommand">
    <PagerStyle AlwaysVisible="true" />
    <MasterTableView DataKeyNames="UniqueID" CommandItemDisplay="Top">
        <Columns>
            <telerik:GridBoundColumn DataField="ID" UniqueName="ID" HeaderText="ID">
            </telerik:GridBoundColumn>
            <telerik:GridTemplateColumn>
                <ItemTemplate>
                    <asp:TextBox ID="TextBox1" runat="server" Text='<%# Eval("Name") %>'></asp:TextBox>
                </ItemTemplate>
            </telerik:GridTemplateColumn>
        </Columns>
    </MasterTableView>
</telerik:RadGrid>
<asp:Button ID="Button1" runat="server" Text="Save All" OnClick="Button1_Click" />

ASPX.CS

public partial class aaaa : System.Web.UI.Page
{

    public List<Employee> lstEmployee
    {
        get
        {
            if (Session["lstEmployee"] != null)
            {
                return (List<Employee>)Session["lstEmployee"];
            }
            else
            {
                return new List<Employee>();
            }
        }
        set
        {
            Session["lstEmployee"] = value;
        }
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            List<Employee> list = new List<Employee>();

            Employee obj = new Employee();
            obj.ID = 1;
            obj.Name = "Name1";
            obj.UniqueID = Guid.NewGuid();
            list.Add(obj);

            obj = new Employee();
            obj.ID = 2;
            obj.Name = "Name2";
            obj.UniqueID = Guid.NewGuid();
            list.Add(obj);

            lstEmployee = list;
        }
    }

    protected void RadGrid1_NeedDataSource(object sender, GridNeedDataSourceEventArgs e)
    {
        RadGrid1.DataSource = lstEmployee;
    }

    protected void RadGrid1_ItemCommand(object sender, GridCommandEventArgs e)
    {
        if (e.CommandName == RadGrid.InitInsertCommandName)
        {
            saveAllData();
            lstEmployee.Insert(0, new Employee() { UniqueID = Guid.NewGuid() });
            e.Canceled = true;
            RadGrid1.Rebind();
        }
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        saveAllData();
    }

    protected void saveAllData()
    {
        //Update Session
        foreach (GridDataItem item in RadGrid1.MasterTableView.Items)
        {
            Guid UniqueID = new Guid(item.GetDataKeyValue("UniqueID").ToString());
            Employee emp = lstEmployee.Where(i => i.UniqueID == UniqueID).First();
            emp.Name = (item.FindControl("TextBox1") as TextBox).Text;
        }
    }

}

public class Employee
{
    public Guid UniqueID { get; set; }
    public int ID { get; set; }
    public string Name { get; set; }
    public bool IsActive { get; set; }
    public int weeknumber { get; set; }
}