put https://yourServer.jamfcloud.com/JSSResource/mobiledeviceextensionattributes/id/
<mobile_device_extension_attribute>
<name>Asset Selector</name>
<description>string</description>
<date_type>String</date_type>
<input_type>
<type>Text Field</type>
</input_type>
<inventory_display>General</inventory_display>
</mobile_device_extension_attribute>