using System.Data.SqlClient;

protected void Button1_Click(object sender, EventArgs e) {
    SqlConnection conn = new SqlConnection();
    conn.ConnectionString= "Data Source=(local);Initial Catalog=TestDb;Integrated Security=True";
    conn.Open();
    int id = GenerateId();
    int parentId = 0;

    if (ASPxTreeList1.FocusedNode != null)
        parentId = (int)ASPxTreeList1.FocusedNode["ID"];

    string textCmd = string.Format("INSERT INTO [Table1] ([ID], [ParentID], [Name]) VALUES ({0},
            {1}, '{2}')", id, parentId, ASPxTextBox1.Text);
    SqlCommand cmd = new SqlCommand(textCmd, conn);
    cmd.ExecuteNonQuery();
    conn.Close();
    ASPxTreeList1.DataBind();
}

<dx:ASPxTreeList ID="ASPxTreeList1" runat="server" AutoGenerateColumns="False"
    DataSourceID="SqlDataSource1" KeyFieldName="ID"
    ParentFieldName="ParentID" Width="307px">
    <Columns>
        <dx:TreeListTextColumn FieldName="Name" VisibleIndex="0">
        </dx:TreeListTextColumn>
        <dx:TreeListSpinEditColumn FieldName="ID" VisibleIndex="1" Visible="False">
            <PropertiesSpinEdit DisplayFormatString="g" NumberFormat="Custom">
            </PropertiesSpinEdit>
        </dx:TreeListSpinEditColumn>
        <dx:TreeListSpinEditColumn FieldName="ParentID" VisibleIndex="1" Visible="False">
            <PropertiesSpinEdit DisplayFormatString="g" NumberFormat="Custom">
            </PropertiesSpinEdit>
        </dx:TreeListSpinEditColumn>
    </Columns>
    <SettingsBehavior AllowFocusedNode="True" />
</dx:ASPxTreeList>