Is the xtype in your pic a CQ.Ext.form.ComboBox or a CQ.wcm.ContentFinder.
If you are using a combo box in a custom xtype - you can use code like:
// create the combo instance
var combo = new CQ.Ext.form.ComboBox({
typeAhead: true,
triggerAction: 'all',
lazyRender:true,
mode: 'local',
store: new CQ.Ext.data.ArrayStore({
id: 0,
fields: [
'myId',
'displayText'
],
data: [[1, 'item1'], [2, 'item2']]
}),
valueField: 'myId',
displayField: 'displayText'
});
The Combo Box API has a default value setting to:
[img]Combobox.png[/img]
We have an article on how to use this ComboBox API within a custom xtype:
http://helpx.adobe.com/experience-manager/using/creating-custom-xtype.html
However - there is no such API that maps to an "xtype tag". You can get a value from a combo box in a custom xtype. Then you can use the AEM TagManager API and create a tag based on that value. See these docs:
http://wem.help.adobe.com/enterprise/en_US/10-0/wem/developing/tagging.html