To set up Envato login button you need to create and account on https://build.envato.com/

if you are an Envato author and want to verify your customers purchases you need to create the app using the same account you use to sell your items
After you login go to My apps -> Register a new app

https://build.envato.com/register 

Give your app a name and select required permissions, see image below

Type your callback URL ( You can find your call back URL on Settings page -> OAuth 2.0 login), Accept terms and conditions then click the Register App button



This is your App secret ( you must copy it before you leave the page )

That's all you have created your Envato app now you can get the App id

To set you Envato login button go to Settings page ->OAuth 2.0 login