I navigated to a webpage and able to add the Username and Password successfully. But I cant seem to invoke a button click.

Can someone please help me with the BUTTON CLICK

HTML Code:
  <form id="auth" action="/nextgen/login" method="post">
                                <fieldset class="placeholder corner blur box-shadow condensed">
                                    <label for="auth-login" class="no-select" data-translate-text="<% wsd.i18n._('Login') %>" unselectable="on"></label>
                                    <input type="text" id="auth-login" name="loginUsername" data-sync="login" autocapitalize="off" spellcheck="false" maxlength="50">
                                </fieldset>
                                <fieldset class="placeholder corner blur box-shadow condensed">
                                    <label for="auth-password" class="no-select" data-translate-text="<% wsd.i18n._('Password') %>" unselectable="on"></label>
                                    <input type="password" id="auth-password" name="loginPassword" autocomplete="off" maxlength="50">
                                </fieldset>
                                <fieldset id="auth-submit" class="blur corner box-shadow">
                                    <input class="corner" type="submit" data-translate-value="<% wsd.i18n._('Log In') %>">
                                </fieldset>
                                <fieldset class="help-links">
                                    <a id="auth-help" href="#" data-translate-text="<% wsd.i18n._('Forgot password?') %>"></a>
                                </fieldset>
                                <fieldset class="help-links">
                                    <a id="system-status" href="https://taxxxxxxxxx-status.hostedtax.xxxxxxxxxx.com/" target="_blank" data-translate-text="<% wsd.i18n._('System Status') %>"></a>
                                </fieldset>
                                <div class="trta1-captcha-container"></div>
                            </form>
Code:

  Private Sub WebBrowser1_DocumentCompleted(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted



        WebBrowser1.Document.GetElementById("loginUsername").SetAttribute("value", "email")
        WebBrowser1.Document.GetElementById("loginPassword").SetAttribute("value", "**")
        WebBrowser1.Document.GetElementById("auth-submit").InvokeMember("post")




    End Sub