Are there websites that allow for a broad search that passes via specific connection airports?
In the case of a one-way flight, I would like to find which flights depart from a certain city (A) and go to anywhere while stopping at city (B). For two-way flights, the return should go back from C to A, where C is what was the selected anywhere destination, possibly with a stopover at a fourth city D.
In other words I want the final destination to be unspecified but to specify the intermediate stops to take advantage of stopovers. This has to be a reasonable route for someone going from A to C, not just a random flight from B onward.
EDIT: I do not want a single use answer but a way to learn how to do this. With my frequent flyer points program, I often have options to include stop-over for no cost. Most times I search for the destination and choose one on the way to back as stop over but often there are no more stop-overs that I have no visited or the destination is too close. Moreover, when redeeming points, the region is offered as a single cost, so it's better if I found a further destination and use my indented location as a stopover.