Hello
You can find it in dhxform_infonote.xml file in form samples
dhtmlxForm/samples/03_api/07_help_info.html
We will add it in documentation in the next update.
<item type="input" name="email2" label="Confirm e-mail" value="" validate="ValidEmail" required="true">
<note>Confirm your valid email address just to be sure it correct.</note>
</item>