How can I get callback data after permission page in unity3D

Question

I tried to programming the Fitbit authorization process but it need some browser issue that I want to avoid them.

Redirect URL: http://localhost

public void GoToAuthorizePage() => Application.OpenURL("https://www.fitbit.com/oauth2/authorize...");

For this purpose, the client must first be referred to the authorize link, and after the permission is confirmed by the user, the authorize code will be sent in the callback header. However, it is difficult for me to get callback information because it's depended on browser behavior, maybe something like this that works..

public static async void GetLocalHostInfo()
{
    var apiPath = "http://localhost/+@info?!!";

    var request = UnityWebRequest.Get(apiPath);

    await SendRequest(request);

    authorizeCode = request.downloadHandler.text.ToAutorizeCode();
}

Type of redirect callback

More information on fitbitautorization: https://dev.fitbit.com/build/reference/web-api/developer-guide/authorization/