preRequestScript

post.js

const postRequest = {
  url: "https://ca6e-2405-4803-fc0e-22e0-54a1-487d-d4c6-ab89.ngrok-free.app/api/auth/login",
  method: 'POST',
  header: {
    'Content-Type': 'application/json',
  },
  body: {
    mode: 'raw',
    raw: JSON.stringify({"username" : "user1234", "password" : "User12345"})
  }
};

pm.sendRequest(postRequest, (error, response) => {
   const res = response.json(); 
   pm.environment.set("accessTokenDemo", res.access_token);
});