Edit in JSFiddle

JavaScript
HTML
CSS
Result

$(function(){


    var $iFrame = $('iframe');
    
    $iFrame.load(function(){
        $('h3').html('Rechner fertig geladen!');
    });
    
    $('h3').html('Lade den Rechner...');
    $iFrame.attr('src', 'https://www.procheck24.de/einsurance/csp/doHomepageEntry.do?cspident=DCD9D250F9F2B1D8BBBDF3AB5D4952B8&p=1');

});

<h3></h3>
<iframe></iframe>

iframe {
    width: 888px;
    height: 800px;
    border: 5px solid red;
}