Here's some sample code from the XML in the feedback widget.
For selecting an entity:
<property key="entity" type="entity" required="false">
<caption>User</caption>
<category>Data source</category>
<description>If this property is set, the username and email address properties can be used to prefill the users identy in the submit form.</description>
</property>
For selecting an attribute on that entity:
<property key="usernameattr" type="attribute" entityProperty="entity" required="false">
<caption>Username attribute</caption>
<category>Data source</category>
<description>This property can be used as username of the user.
</description>
<attributeTypes>
<attributeType name="String"/>
</attributeTypes>
</property>