How to make portal_actions use a popup window

I have been curious how the "Contact" site action at the bottom of every Plone site was made to display the contact form in a popup window.

This line is how it's done:

<property name="modal" type="text">{}</property>

To make the "Accessibility" info display in a popup window, you

• go to the ZMI -> portal_actions -> site_actions -> accessibility (e.g. /portal_actions/site_actions/accessibility/manage_propertiesForm)
• at the bottom of the page, in the Name field enter "modal", choose "text" for the Type, and enter "{}" for the Value, then press Add