If it does not already exist, add the key <add key="ServiceCalendarCustomProperties" value="prop1,prop2" /> to the SyncProcessConfig.xml file, which is located in the installation folder.
In the value, replace prop1,prop2 with the property names, using a comma to separate the properties.
You can only use properties that are available on the FSSchedules IDO. If needed, add user-defined properties to the IDO on the IDO Properties form.
Modify the HTMLEmailBody.html or HTMLEmailBodyMisc.html template to include any new properties. The properties should be used inside a bracket, when used inside the template.
Save your changes and restart the background task.