There are different ways to force an app to use MPTCP, where the most convenient method is 'mptcpize run <cmd>', see: https://www.mptcp.dev/setup.html#force-applications-to-use-m...

But the best is to let the app (and their users) controlling that, with a nice option. With Chrome/Firefox/..., we could enable MPTCP per domain for example.