How do I simulate Text input to a WPF TextBox?

I want to simulate user input to a WPF TextBox. I want to input a character such that the OnPreviewTextInput event is triggered. I tried setting the Text through the Text property, but this didn't trigger the event:

public void SomeFunction()
{
    var textBox = new TextBox();
    textBox.Text = "A";                     
}

Can I trigger the event explicitly somehow?