<telerik:RadDropDownTree ID="DdtDpt" runat="server" CheckBoxes="CheckChildNodes" CheckChildNodes="false" CheckNodeOnClick="true" 
    DataFieldParentID="pid" DataFieldID="id" DataTextField="name" DataValueField="id" Width="350px">
</telerik:RadDropDownTree>

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        DdtDpt.DataSource = GetDummyDepartments();
        DdtDpt.DataBind();
    }
}

private List<Department> GetDummyDepartments()
{
    return new List<Department>
    {
        new Department { id = 1, pid = null, name = "Head Office" },
        new Department { id = 2, pid = 1, name = "HR" },
        new Department { id = 3, pid = 1, name = "IT" },
        new Department { id = 4, pid = 3, name = "Development" },
        new Department { id = 5, pid = 3, name = "Support" },
        new Department { id = 6, pid = null, name = "Branch Office" },
        new Department { id = 7, pid = 6, name = "Sales" },
        new Department { id = 8, pid = 6, name = "Logistics" }
    };
}


public class Department
{
    public int id { get; set; }
    public int? pid { get; set; }
    public string name { get; set; }
}