VS 2008 - Webbrowser - Click A Button After Filling Out A Form

I am trying to click a button after filling out a form in vb.net 2008. I have the following code but I cannot seem to get it to work.

WebBrowser1.Document.All("username").InnerText = TextBox1.Text
WebBrowser1.Document.All("password").InnerText = TextBox2.Text
WebBrowser1.Document.GetElementById("submit_button").InvokeMember("submit")

I am trying to fill in a simple login form and click submit. I am able to copy the information into the username and password fields but cannot seem to get it to click submit within the code.

Here is the html code for the submit button below:

<div id="submit_button">
<button type="submit" value="Login Now!" onmouseover="this.style.backgroundPosition='bottom';" onmouseout="this.style.backgroundPosition='top';" onclick="return SetFocus();">Login Now!</button>
</div>

Why this isn't working I cannot seem to figure it out.