to dogleg a route, click on the route then click on the waypoint by which you want to break it
What's "obvious" is what people are used to.
What people are used to is Google maps.
A similar interface would be to drag a segment of the route, which would rubber-band it as two segments, and you could then drop the new vertex onto a waypoint.
If indeed you really need to create the waypoints first; people might like to just drag bits of their route around to avoid CAS, glider sites etc without having to do a separate operation to create a new waypoint for each vertex. Even better might be to auto-create new waypoints for each vertex, present them suitably in the plan (eg as bearing/distance from a VOR/DME, something that's easy to enter into a GPS), but not persist them other than with the particular route.