Most will let you do it two ways:
1) Click on the start point and then click on the next point, software will route it, then click on the next point, software will route it, repeat until you get to the end.
2) Click on the start point, click on the end point, drag the route to each point in between.
Strava does it both ways, I think ridewithgps does both (option 2 you might need to add a point on the existing route and then drag it rather than just dragging the route)
cycletravel does 2) bit I don't think it does 1)
Google maps will do 2) not sure about 1)
I'm assuming you are doing this on a computer.