@Peter-Dorr-0 In the route planner, open the context menu of the waypoint where you want to start. Then, click on the three dots and select the flag with 'Go.'
waypoint_context_menu.png
If you find this too complicated and want to start driving immediately, you can press and hold the waypoint where you want to start in the navigation app, starting from your address. That will become the point you are guided to, and from there, the app will pick up the route on its own. However, you'll miss the first part of the route. You will still finish the route at the endpoint.