To set a start (or ending) point, you must edit the data of your vehicles, otherwise the start and end point will be conditioned by the direction of your stops.
You have 2 ways to do this:
To make a change and have it reflected in all your future plans, you must do it from the Vehicles section inside Drivers.
If, on the other hand, you only want to modify the start/arrival point for a specific plan, you must do it from the plan itself.
In both cases, double-clicking on the vehicle will open a right panel. There, you will have to edit the Start point and Arrival point fields. With this change, your drivers will start and end their routes in the direction you have defined.
IMPORTANT: If you want your vehicles to start and end their route in the same direction, you must activate the Circular Route checkbox.