https://YOUR_DOMAIN/authorize? for Authorization Code Flow returns Not Found

As a test automation engineer I want to get access token for different app users in order to be able to send API requests to my app.

From UI page I see our app sends request to (1)auth0/usernamepassword/login and then to (2)auto0/oauth/token

But I can’t figure out how this 2 calls are connected (can’t see AUTHORIZATION_CODE from response of the first request)

I went thru auth0 docs and found this tutorial Call Your API Using the Authorization Code Flow

But calling https://YOUR_DOMAIN/authorize?.. as a POST request returns Not found, as a GET request returns html page like this

"\n\n\n Sign In with Auth0\n <meta charset=“UTF-8” />\n <meta name=“viewport” content=“width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0” />\n <meta http-equiv=“X-UA-Compatible” content=“IE=edge,chrome=1”>\n <meta name=“robots” content=“noindex, nofollow”>\n <link rel=“shortcut icon\ …”

I just felling a bit frustration with authorization.
My end goal is to get Access token for different login (user\password credentials) to be used in API calls.
Can someone please clarify and forward me towards the correct path?