Yes I think you can do that with Finch. I used it in the past for some project.

You can use Finch.build to prepare your request. I don’t know if this changed but basically this is how it worked for me :

Finch.build(
        :post,
        some_url(),
        some_headers(),
        the_body()
      )
|> Finch.request(MyFinch)

• First we have the type of request as an atom.
• some_url() return the request url as string.
• some_headers() is a list of tuples. For example [{"Content-Type", "application/json"}, ...]
• the_body() is the data you want to send. I think it should be iodata. In my case I used a map that is converted to iodata. If I take your curl example it would look like below :

%{auth: "123abc", id: 1, var: "foo"}
|> Jason.encode_to_iodata!()

Edit:
The response for this request could be {:ok, %Finch.Response{body: body}} or {:error, message}