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");

//危険口座初期画面
HtmlElementCollection forms = webBrowser1.Document.All;
forms["string"].SetAttribute("value", T_dandya.Text);

HtmlElement form = webBrowser1.Document.Forms[0];
form.InvokeMember("submit");

//画像がない場合
if (var.nokoriFileCount == 0)
{

HtmlElement usernameButton = webBrowser1.Document.GetElementById("auc_submit2");

usernameButton.InvokeMember("click");

}
else //画像がある場合
{

HtmlElementCollection buttons = webBrowser1.Document.GetElementsByTagName("input");

foreach (HtmlElement element in buttons)
{
if (element.GetAttribute("value") == "画像登録画面へ")
{

element.InvokeMember("click");

}
}

}