"The calling thread must be STA" workaround

I know there are a few answers on this topic on SO, but I can not get any of the solutions working for me. I am trying to open a new window, from an ICommand fired from within a datatemplate. Both of the following give the aforementioned error when the new window is instantiated (within "new MessageWindowP"):

Using TPL/FromCurrentSynchronizationContext Update: works

public class ChatUserCommand : ICommand
{
    public void Execute(object sender)
    {
        if (sender is UserC)
        {
            var user = (UserC)sender;
            var scheduler = TaskScheduler.FromCurrentSynchronizationContext();                  
            Task.Factory.StartNew(new Action<object>(CreateMessageWindow), user,CancellationToken.None, TaskCreationOptions.None,scheduler);         
        }
    }

    private void CreateMessageWindow(object o)
    {
        var user = (UserC)o;
        var messageP = new MessageWindowP();
        messageP.ViewModel.Participants.Add(user);
        messageP.View.Show();
    }
}

Using ThreadStart: Update: not recommended, see Jon's answer

public class ChatUserCommand : ICommand
{
    public void Execute(object sender)
    {
        if (sender is UserC)
        {
            var user = (UserC)sender;

            var t = new ParameterizedThreadStart(CreateMessageWindow);
            var thread = new Thread(t);
            thread.SetApartmentState(ApartmentState.STA);
            thread.Start(sender);           
        }
    }

    private void CreateMessageWindow(object o)
    {
        var user = (UserC)o;
        var messageP = new MessageWindowP();
        messageP.ViewModel.Participants.Add(user);
        messageP.View.Show();
    }
}

Thanks

EDIT. Based on the responses so far, I'd like to point out that I have also tried BeginInvoke on the current dispatcher, as well as executing the code in the original method (that's how the code started). See below:

BeginInvoke Update: not recommended see Jon's answer

public class ChatUserCommand : ICommand
{
    public void Execute(object sender)
    {
        if (sender is UserC)
        {
            var user = (UserC)sender;
            Dispatcher.CurrentDispatcher.BeginInvoke(new Action<object>(CreateMessageWindow), sender);       
        }
    }

    private void CreateMessageWindow(object o)
    {
        var user = (UserC)o;
        var messageP = new MessageWindowP();
        messageP.ViewModel.Participants.Add(user);
        messageP.View.Show();
    }
}

In same thread Update: works if you are on UI thread already

public class ChatUserCommand : ICommand
{
    public void Execute(object sender)
    {
        if (sender is UserC)
        {
            var user = (UserC)sender;
            var messageP = new MessageWindowP();
            messageP.ViewModel.Participants.Add(user);
            messageP.View.Show();    
        }
    }

}

BeginInvoke, using reference to dispatcher of first/main window Update: works

 public void Execute(object sender)
   {
       if (sender is UserC)
       {
            var user = (UserC)sender;
                    GeneralManager.MainDispatcher.BeginInvoke(
                               DispatcherPriority.Normal,
                               new Action(() => this.CreateMessageWindow(user)));      
        }
    }

where GeneralManager.MainDispatcher is a reference to the Dispatcher of the first window I create:

     [somewhere far far away]
        mainP = new MainP();
        MainDispatcher = mainP.View.Dispatcher;

I'm at a loss.

The calling thread must not only be STA, but it must also have a message loop. There's only one thread in your application that already has a message loop, and that's your main thread. So you should use Dispatcher.BeginInvoke to open your window from your main thread.

E.g. if you have a reference to your main application window (MainWindow), you can do

MainWindow.BeginInvoke(
    DispatcherPriority.Normal, 
    new Action(() => this.CreateMessageWindow(user)));

Update: Be careful: you cannot blindly call Dispatcher.CurrentDispatcher because it doesn't do what you think it does. The documentation says that CurrentDispatcher:

Gets the Dispatcher for the thread currently executing and creates a new Dispatcher if one is not already associated with the thread.

That's why you must use the Dispatcher associated with an already-existing UI control (like your main window as in the example above).