Geckofx: iframe with session token

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Gecko

I have a Geckofx webcontrol in my winforms app. I want to fill some textboxes from my code. The website, which I want to fill is devided in iframes. The iframe I want to get into has as source a link with a session ID.

It seems to be not possible to go into that iframe en get its content:

GeckoIFrameElement editFrame = null;
        GeckoIFrameElement mainFrame = geckoWebBrowser1.Document.GetElementsByTagName("iframe")[0] as GeckoIFrameElement;
        GeckoIFrameElement workFrame = mainFrame.ContentDocument.GetElementsByTagName("iframe")[3] as GeckoIFrameElement;
        editFrame = workFrame.ContentDocument.GetElementById("iframePage1") as GeckoIFrameElement;

The workFrame.ContentDocument returns null.

Is there any possibility to get this work ?

What I have tried:

I tried to get the ContentDocument from the iframe with the sessiontoken.