Creating OAuthSession object using .Net api to access Box api

I am using .Net Box api to access the box folders and libraries. I am using the below code:

Uri Redirect_Uri = new Uri("http://localhost");
var config = new BoxConfig(CLIENT_ID, CLIENT_SECRET, Redirect_Uri);
var session = new OAuthSession(ACCESS_TOKEN, REFRESH_TOKEN, 3600, "bearer");
var client = new BoxClient(config, session);

How to get the ACCESS_TOKEN and REFRESH_TOKEN in OauthSession object.