Unfortunately the official Twitter Counter is no longer active from November 20, 2015 (Official announcement by Twitter).
But you can use an unofficial counter that counts the number of shares on twitter. Since version 2.5.4, Cresta Social Share Count, has added support for newsharecounts.com API and opensharecount.com API.
To use Twitter Counter with Cresta Social Share Counter PRO, just read the official guide by NewShareCounts.com in this page.
Or you can use OpenShareCount.com API’s, just go in this website, login with your Twitter account and add your website URL.