18.3.4 + Erlang TLS1.2 [+ IIS] Issues

Hackney expects SSL options to be passed in the :ssl_options key. See https://github.com/benoitc/hackney/blob/master/doc/hackney.md#request-5

Try this:
OAuth2.Client.get_token(..., [ssl_options: [versions: [:'tlsv1.2']]])

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