The easiest way is to choose “Edit Mode” by pressing the “Edit” button on the top left side of the window, and then right-clicking on the chart at the location you wish to start the route. This will add the first route point at that location. You can right click on that route point to type in different lat/lon coordinates, if you’re not happy with its location. To add additional points to a route, move the cursor near an endpoint of the route and hold down the left mouse button. A new routepoint will appear, connected to the current endpoint of the route. Move the new routepoint to the desired location, and release the left mouse button. This will place the new routepoint, which becomes the new endpoint.
There are a number of other ways to add new points to a route. Please check the documentation.