Basic-Auth against Proxy, then open Website behind that Proxy

Hi all,

we want to perform a basic authentification against a proxy server. This is already working. When the authentification was successfully i want to open a webpage within an iframe or in the inApp Browser plugin from Capacitor. The webpage shows “unauthorized” - i think the context or something else is not the same?

proxyAuthentification(user, password): Promise<any> {
        const url = 'https://connect.deos-ag.com';
        const headers = new Headers();
        headers.append('Authorization', 'Basic' + btoa('USER' + ':' + 'PASSWORD'));

        const base64Auth = this.nativeHttp.getBasicAuthHeader(user, password);

        return new Promise((resolve, reject) => {
            this.nativeHttp.get(url, {}, base64Auth)
                .then(data => {
                    console.log('data:', data);
//                    NOW OPEN A SPECIFIED WEBSITE BEHIND THAT URL
                    resolve(data);
                }, (err) => {
                    console.log('error status', err.status);
                    console.log('error message', err.message);
                    console.log('error headers', err.headers);
                    reject(err);
                });
        });
    }

How could we achieve the desired behaviour?