To run mutations in the GraphiQL interface, click on "HTTP HEADERS" at the bottom of the page.
Add new header "authentication", plus the secret key (cf. config.exs)
Run
iex --sname node_a@localhost -S mix phx.server
in the first terminal.
In the second terminal, run
PORT=4001 iex --sname node_b@localhost -S mix phx.server