So as far as I can tell, you’re basically trying to call an endpoint in an existing Elixir application? If so, there’s a number of HTTP clients that you can utilise:

• :httpc is provided by the Erlang runtime. While it does work, there’s a lot of Erlang idioms that you’ll need to use.
• hackney is a pretty common HTTP client written for Erlang. It’s a fairly used alternative to httpc.
• mint is probably the most recent HTTP client on the block, written in Elixir.

However, I would make the distinction between the HTTP client library as wrappers that use said client libraries/abstractions. One such example is finch (GitHub - sneako/finch: Elixir HTTP client, focused on performance · GitHub) which utilises mint which is focused on performance.

If you want something even more generalised, Tesla (GitHub - elixir-tesla/tesla: The flexible HTTP client library for Elixir, with support for middleware and multiple adapters. · GitHub) can be used with various HTTP clients (finch, hackney, httpc), that includes abstractions such as middleware and a more module definition approach to client design.

Keep in mind that these assume “normal” HTTP requests. If you need something to work with Phoenix channels, I recall an Elixir library for that, but it escapes me at the moment.