dialog.xml

<?xml version="1.0" encoding="UTF-8"?>
<jcr:root xmlns:cq="http://www.day.com/jcr/cq/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0" xmlns:nt="http://www.jcp.org/jcr/nt/1.0"
    jcr:primaryType="cq:Dialog" xtype="dialog">
    <items jcr:primaryType="cq:WidgetCollection">
        <tabs jcr:primaryType="cq:TabPanel">
            <items jcr:primaryType="cq:WidgetCollection">
                <first jcr:primaryType="nt:unstructured" title="Form" xtype="panel">
                    <items jcr:primaryType="cq:WidgetCollection">
                        <name jcr:primaryType="cq:Widget" fieldLabel="Name" name="./name" xtype="textfield" allowBlank="{Boolean}false"
                            validator="function(value) {
                                var dialog = this.findParentByType('dialog');

                                var url = CQ.HTTP.addParameter(dialog.path + '.validator.json', 'value', value);

                                var result = CQ.HTTP.eval(url);

                                return result.valid ? true : 'Form name already exists.';
                            }" />
                    </items>
                </first>
            </items>
        </tabs>
    </items>
</jcr:root>