The WebBrowser control has a Document property, which returns an HtmlDocument. The HtmlDocument has several members you can use to traverse and manipulate the DOM.
Once you've used these methods to find the form, you can use InvokeMember to call the form's submit method.
If you know the page has a single form:
foreach (HtmlElement form in webBrowser1.Document.Forms)
form.InvokeMember("submit");
If you know the ID of the form you would like to submit:
HtmlElement form = webBrowser1.Document.GetElementById("FormID");
if (form != null)
form.InvokeMember("submit");
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