The question is not clear enough.  
You appear to be trying to use jQuery to do *explicit* OAuth from a web page.  If so, **this is not allowed.**  

Explicit OAuth is for servers.  You are not running that code in node.js are you?  

**Anything that runs in the user's browser, like javascript, needs to use ["implicit" (Client side) OAuth][1].**  Implicit OAuth was developed for that purpose.

[This answer][2] gives the **setup and steps needed** to use the implicit (Client side) OAuth flow.

[This answer][3] gives working code to **implement implicit OAuth in a userscript**, if you are trying to do that.  (Not the same as a web page's javascript.)


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**If you have your own server**, and control the webpage(s), then you *would* use explicit OAuth.  But you need to do it properly (no client-side AJAX for the OAuth bits).  That includes:  when your **server** POSTS to `stackexchange.com/oauth/access_token` it must do it as though it is browsing that page.

  [1]: https://api.stackexchange.com/docs/authentication
  [2]: https://stackapps.com/a/7010/7653
  [3]: https://meta.stackexchange.com/a/293498/148310