OAuth2如何重定向缺少占位符？

I created an APP on BufferApp.

In redirect URL I did insert my main domain (http://www.example.com/get_token).

In this mode I could re-use same APP for several website, with different URIs, without necessity to create an APP for every website or copy/paste token(s) (in Oauth2 you need to specify the redirect URI).

For Facebook, Twitter, Instagram and some others I follow same pattern.

NEW_SITE_TO_AUTHORIZE = > 
Oauth2 Provider = > 
MAIN and DIFFERENT DOMAIN THE ONLY ONE WITH THE LOGIC = >
redirect to NEW_SITE_TO_AUTHORIZE

transfering tokens via GET. Some service provide some "placeholder" in which I insert the URL of the "NEW SITE TO AUTHORIZE" and redirect to it after getting the code.

With BufferApp I did try to append another querystring ("&return=...") but it lost "on the way".

Do you have some idea to bypass this problem?

OK, I found a GET "state" that mantains value between domains. I write here for someone with same problem.

So url here:

GET https://bufferapp.com/oauth2/authorize?
    client_id=...&
    redirect_uri=...&
    response_type=code

could be:

GET https://bufferapp.com/oauth2/authorize?
    client_id=...&
    redirect_uri=...&
    response_type=code&
    state=RETURN_URL