The @@config@@ construct is a placeholder that will be replaced by the runtime. The documentation you linked to shows how you can include that placeholder in the hosted login page and get a Javascript object from the value that will be provided at runtime. The information contained within this value will be provided automatically by the runtime based on your configuration and also possibly based on parameters received from the client application.
You don’t directly control most of the contents within this object, however, as mentioned in the linked documentation, if you pass additional query string parameters when initiating the navigation to the hosted login page from your client application then these extra parameters will be available at config.extraParams; assuming you processed the contents of the @@config@@ into a variable named config like it is shown in the documentation:
var config = JSON.parse(decodeURIComponent(escape(window.atob('@@config@@'))));
From your post it seems your goal is to pass additional parameters so you should focus on the contents of config.extraParams after ensuring that you pass the additional data through query string parameters.