As with all network code, get it working with Postman or curl first. That makes it NOT a Unity problem. Solve the networking first. You must solve that or anything else is irrelevant.
Once you have that working, then try and port it to UnityWebRequest. If it still doesn’t work, one handy debug solution is to hook up a proxy like Charles and compare the functioning output from Postman / curl to the output from Unity, and from that reason about where your issues are.