Ribbon Button in List, which sends Data to C# code

First, I'm not really a developer, but since I'm the only "sharepoint-guy" in my company I need to learn this, too. So here's my problem: I need a ribbon button, which sends data of the marked item to a C# application. This works perfectly fine with the context menu, but gives me a script error on the ribbon.

This is my working code for the context menu:

<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://ift.tt/sQmbje">
  <CustomAction Id="SendTest"
                Location="EditControlBlock"
                Title="Sent Test"
                RegistrationType="List"
                RegistrationId="100"
                ImageUrl="/_layouts/IMAGES/DOCLINK.GIF">
    <UrlAction Url="javascript:__doPostBack('SendTest',{ItemId});"/>
  </CustomAction>
</Elements>

This is the erroneus code for the ribbon:

<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://ift.tt/sQmbje">
  <Control
     ControlAssembly="SPTest.CustomMenuItem, Version=1.0.0.0, Culture=neutral, PublicKeyToken=8b4e18645f42d2fd"
         ControlClass="SPTest.CustomMenuItem.CustomItemAction" Sequence="50" Id="AdditionalPageHead"/>
  <CustomAction
  Id="SendToWCF"
  RegistrationType="List"
  RegistrationId="100"
  Location="CommandUI.Ribbon.ListView"
  Rights="EditListItems">
    <CommandUIExtension>
      <CommandUIDefinitions>
        <CommandUIDefinition
         Location="Ribbon.ListItem.Actions.Controls._children">
          <Button
           Id="SendToWCF.SendButton"
           Alt="Send To WCF"
           Sequence="1"
           Image32by32="/_layouts/Images/search32x32.png"
           Command="SendToWCFCommand"
           LabelText="Send to WCF"
           TemplateAlias="o1"
           CommandType="General"/>
        </CommandUIDefinition>
      </CommandUIDefinitions>
      <CommandUIHandlers>
        <CommandUIHandler
         Command="SendToWCFCommand"
         CommandAction="javascript:__doPostBack('SendToWCF',{ItemId});" />
      </CommandUIHandlers>
    </CommandUIExtension>
  </CustomAction>
</Elements>

This is my c# (for testing) - I replace the SendTest with SendtoWCF for the ribbon:

using System.IO;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;
using System.ServiceModel;

namespace SPTest.CustomMenuItem
{
    public class CustomItemAction : SPLinkButton
    {
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            if (this.Page.Request["__EVENTTARGET"] == "SendTest")
            {
                int itemId = Convert.ToInt32(this.Page.Request["__EVENTARGUMENT"]);

                System.IO.TextWriter writer = new StreamWriter(@"C:\\tmp\\custommenuoutput.txt", true);

                writer.WriteLine("Event Fired at:" + DateTime.Now.ToLongTimeString() + ": Item ID:" + itemId.ToString());

                writer.Close();
            }

        }
    }
}

When I use the context menu it works fine. this.Page.Request["__EVENTTARGET"] gets the correct value and the file is written to C:\tmp

When I use the ribbon I get a javascript error and __EVENTTARGET is still null:

Unhandled exception at line 359, column 21 in http://spdev/_layouts/15/sp.ribbon.debug.js?rev=lltVSagCIwhL+jnBu7MRag==

0x800a03eb - JavaScript runtime error: Expected ':'

Am I missing something. Do I have to tell him somehow, which Item he should use? Because at the moment it doesn't make a difference if none, one or more items are selected. Can this be the problem?