How do I get GraphiQL (with Phoenix & Absinthe) to configure a websocket connection for a subscription in advanced mode?

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I have GraphiQL (running in Chrome on macOS) in advanced mode setup with my Elixir/Phoenix/Absinthe web server. It works great for queries and mutations, but when I try a subscription I get: “Subscriptions cannot be run over HTTP. Please configure a websocket connection”:

It works with GraphiQL in simple mode:

And it works with GraphiQL in playground mode:

Is this a bug in advanced mode? Or am I failing to configure it properly?


Have you ever get to solve this/ configure the and serve interface: :advanced properly for subscriptions?

EDIT: found this documentation for setting up sockets:

GraphiQL works fine in advanced mode for subscriptions in Safari (on macOS), so I got in the habit of using using GraphiQL in Safari and gave up on Chrome.