How do I select dropdown checkboxes in html with c# and webkit-browser (javascript)?

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C#

Javascript

HTML

WebKit

Hi,
I'm working with the WebKitBrowser in C# (Visual Studio 2010) and I want to automatically create a user in the web application of owncloud.
The login form is displayed in Picture 1. By clicking "Gruppen" a dropdown field with checkboxes appears (Picture 2). The html-code is below:

<form id="newuser" autocomplete="off">
<input id="newusername" type="text" placeholder="Loginname" /> 
<input type="password" id="newuserpassword" placeholder="Passwort" /> 
<select class="groupsselect" id="newusergroups" data-placeholder="groups" title="Gruppen" multiple="multiple">
	<option value="Benutzer">Benutzer</option>
	<option value="admin">admin</option>
</select> 
<input type="submit" value="Anlegen" />
</form>

I'm able to fill the newusername and newuserpassword textboxes and submit the form, for example:

String javascript = "document.getElementById('newusername').value = '" + newusername + "';";
WebKitBrowser1.StringByEvaluatingJavaScriptFromString(javascript);

But I'm not able to select a value in newusergroups (Picture 2), for example:

String javascript = "document.getElementById('newusergroups').options[1].selected = true;";
String javascript = "document.getElementById('newusergroups').value = 'admin';";

Maybe someone can help me? Thanks in advance and best regards,

Oli