Adding dependencies on your visual is not yet possible in the Office Timeline add-in. However, you can use PowerPoint's connectors as a workaround to show how your tasks depend on one another. Here's how to do it:
Go to the Insert tab in PowerPoint and click Shapes.
From the menu that pops up, under the Lines category, choose the type of connector you'd like to use. All line styles except for the last 3 listed work as connectors.
Now hover your mouse over the first task you wish to link the connector to. You'll notice connection dots will appear over that task.
Click the dot you want to attach the connector to and then drag it to the second task you wish to link.
That's it. The connector created will stay attached to the linked tasks and move together with them even when you change their dates or position on the timeline.