The Mendix calendar widget is a wrapper on the jquery fullcalendar widget. It looks like the property you need to specify is scrollTime, which unfortunately doesn't look like a current available property in the latest revision of the Mendix calendar widget.
I went ahead and added the option to the Mendix widget and created a pull request on GitHub. The owners of the repository will review and perhaps merge this into the official widget.
For some reason, the scrolling doesn't work when you first load up a calendar, but it does work when you change views. For example, loading the calendar directly to an agenda won't scroll to the specified time. However, if you start the calendar in a different view, then jump to an agenda, the scrolling does work. Hopefully someone can review this and perhaps find a solution.
It will probably be a few weeks before we could see this make its way to the app store. In the meantime, here's a modified widget if you want to try it - note this build is not officially supported by Mendix.