Text.Suggest in XML format

By default Text.Suggest expects data in JSON format.
webix_debug.js, v1.6, line 2001: webix.DataDriver.json.toObject()

How can I use Suggestions in XML format?

Same as for data components, you can use datatype property, to define the type of data.
http://webix.com/snippet/8ea8bcca

I use suggestions list from external XML: http://webix.com/snippet/75f6558f

Where “http://192.168.0.86/DropdownDataGenerator.ashx?DropdownID=4” returns data like:

"<data><item value='123'/><item value='321'/></data>"

Is there any way to define that url returns data in XML?

Yes, you can use the “datatype” in case of init from xml as well

<sctext id="minWidth_as_Text" name="minWidth_as_Text" label="Min Width (with suggest)">
  <config name="suggest" url="http://192.168.0.86/DropdownDataGenerator.ashx?DropdownID=4" datatype="xml" />
</sctext> 

http://webix.com/snippet/9b4e58a5