I have a Textbox and need to press the right button and click paste, it checks, tried several methods like
TxtTexto.PreviewMouseLeftButtonDown+= new MouseButtonEventHandler(OnMouseClick);
TxtTexto.MouseUp += new MouseButtonEventHandler(OnMouseClick);
TxtTexto.PreviewMouseDown += new MouseButtonEventHandler(OnMouseClick);
TxtTexto.MouseDown += new MouseButtonEventHandler(OnMouseClick);
Among other codes, is it possible to capitalize on this event? 
Try something like this.
public Window1()
{
InitializeComponent();
// "tb" é o seu TextBox
DataObject.AddPastingHandler(tb, OnPaste);
}
private void OnPaste(object sender, DataObjectPastingEventArgs e)
{
//faça suas verificações aqui use o 'e' para obter os dados presente no past.
var isText = e.SourceDataObject.GetDataPresent(DataFormats.UnicodeText, true);
if (!isText) return;
var text = e.SourceDataObject.GetData(DataFormats.UnicodeText) as string;
}
References link