To include an OnStage login box on your site, you can use the sample code from this page and style it however you wish. In the code below, replace sitename with the URL of your OnStage site.
<form action="https://sitename.onstageportal.com/LoginDispatch.aspx"
method="post" name="form1">
Email: <input type="textbox" name="username" id="username" /><br
/>
Password: <input type="password" name="password" id="password" /><br
/>
<input type="submit" value="Login" />
</form>