I want to find the cheapest flight options between two cities but I'm not able to transit through some countries and I know at least a couple of airlines have an option through a particular third city.
But I don't know which, if any, flight search engines can search one specific but non-direct route.
Is there a way to do this?
(I'm looking for a general method but if you need an example, I'd be going from Beijing to Mexico via Vancouver. I specifically cannot go via any US airport.)