I can't change username with API.

I'm creating an app that allows me to change my Minecraft username without opening a website.

There are two steps to complete this app, the first is to get an AccessToken. The second is to use the AccessToken to send a name change request.

I was able to get an AccessToken by sending a request to https://authserver.mojang.com/authenticate. However, if you try to change the name using this AccessToken, the error

{"error":"Unauthorized","errorMessage":"The request requires user authentication"}

will be returned.

I tried using the Access Token saved in the cookie of Google Chrome and it succeeded.
I want to change the name by using the AccessToken obtained from https://authserver.mojang.com/authenticate. Is there any improvement plan?

( Sorry for bad English. )