Clear AjaxControlToolkit ComboBox

I have a ComboBox control from AjaxControlToolkit, it works fine. But I can not clear selected item to empty (when page is load, it show nothing, and then I can either click dropdown to pick an item or type in an item) I have button and I want to clear my select after click the button. I tired several ways but neither works.
My ComboBox like this:
<asp:ComboBox ID="ComboBox1" runat="server" AutoCompleteMode="Suggest" DataSourceID="SqlDataSource1" DataTextField="Name" DataValueField="NameID" EnableViewState="False" MaxLength="0" style="display: inline;" Width="86px"></asp:ComboBox>
and DropDwonStyle property is default (DropDown).
I tried:
1. ComboBox1.SelectedIndex = -1
2. ComboBox.SelectedItem.Text = ""
3. ComboBox1.SelectedValue = -1
4. ComboBox.Item.Clear()
Is There a ...