This example shows how to embed TreeView in a ComboBox template.

<%
 Ajax.Dna().ComboBox("combox1")
                .Template(()=>{
                // Add any controls or html elements here for your needs.
                %>
                
                <%
                })
                .Render();
%>

The Template() method allows you embed any control in it. When combobox is dropdown you could use the combo jquery method to set the combobox selected value and close it.

      $("#combo1").combo("setValue","Your Selected Text","Your Selected Value");