I am sorry for asking this standard question but the documentation didn't help me getting a working implementation.
I have installed the Iframe extension successfully and added this server parameter to my LocalSettings.php file:
$wgIframe['server']['arte'] = [ 'scheme' => 'https', 'domain' => 'arte.tv' ];
Then I have added an iframe tag to my test page. It reads:
<iframe key="arte" level="www" path="player/v7/index.php?json_url=https%3A%2F%2Fapi.arte.tv%2Fapi%2Fplayer%2Fv2%2Fconfig%2Fde%2F089058-000-A&lang=de&autoplay=false&mute=0&previewData=%7B%22title%22%3A%22Jungenbeschneidung%22%2C%22subtitle%22%3A%22Mehr%20als%20nur%20ein%20kleiner%20Schnitt%22%2C%22image%22%3A%22https%3A%2F%2Fapi-cdn.arte.tv%2Fimg%2Fv2%2Fimage%2FddqytqyeNjsuL3b3Jjt7N3%2F940x530%22%7D" style="transition-duration:0;transition-property:no;margin:0 auto;position:relative;display:block;background-color:#000000;" frameborder="0" scrolling="no" width="100%" height="100%" allowfullscreen="true" />
I don't know if this is the right way to use this.