User auth information is stored in cookies. Browser protects cookies from access by other websites. So your web service cannot get login information, because it has another origin and another browser session.
May be it’s possible if you somehow join the Webstation web app and your web service in a one origin (host and port).
Integration with Grafana is similar. Grafana dashboards have URL with long GUID. Depending on the settings, they can be open separately by a user who knows that URL.
Rapid SCADA web page can interact with your page by sending HTTP requests. POST requests more hidden from a user comparing with GET requests. For example, your page may require special POST request before providing data.