How to add Google App Script as a gadget to Google Calendar?

Question

I created a small App Script with a small UI. Now I'd like to have it as a gadget in my Google Calendar.

I tried to embed a gadget with the following basic XML:

<Module>
<ModulePrefs title="Reminder" height="800"  scrolling="true"/>

<Require feature="dynamic-height"/>
<Require feature="settitle"/>
<Optional feature="maximize"/>

<Content
 type="url" href="..."
 />
</Module>

For the href URL I tried both the Google Site Web page with the script gadget and the direct like to the gadget published as a service. Nothing works, the UI does not appear.