Please do not assume that anyone does know what a certain snippet in a foreign language does. Please try to explain how the request should look like.

HTTPoison is cool but take a look at Tesla also.

Link: https://github.com/teamon/tesla
Documentation: https://hexdocs.pm/tesla/api-reference.html

HTTPoison POST Example. Auth goes into the trailing options.
post!(url, body, headers \\ [], options \\ [])

iex(7)> {code, response} = HTTPoison.post "https://bd-partner-a-system.blackboard.com/learn/api/public/v1/oauth2/token", "grant_type=client_credentials", [{"Content-Type",  "application/x-www-form-urlencoded"}], [hackney: [basic_auth: {"user", "password"}] ]

The code is :ok, or not. The response will be an %HTTPoison.Response

HTTPotion POST Example. Most everything, including auth, goes into options.
response = HTTPotion.post(url, options \\ [])

iex(2)> options =  [body: "grant_type=client_credentials",
     headers: ["Content-Type": "application/x-www-form-urlencoded"],
     basic_auth: {"user","password"}]
iex(3)>response = HTTPotion.post("https://bd-partner-a-system.blackboard.com", options)

The response will be an %HTTPotion.Response.

Docs are easily found with a search. Between the docs you find and the above examples, you should be set!