I would first try and reproduce your Node.JS code with curl on the command line. It helps establish a baseline with which to compare.
That being said, I noticed that in your Node code you have process.env.USERNAME as a value for "identifiant" and that you hardcoded "" secret for its value in the Elixir code. Are both identical? (Same goes for "password"). Of course, don’t post users and passwords here, I am just making sure.
