public partial class Messagebox
{
    public Messagebox()
    {
        InitializeComponent();
    }
    public void Show(string title, string text)
    {
        Show();
        titleE.Content = title;
        textE.Content = text;
    }
}

public Messagebox Messagebox = new Messagebox();
    public MainWindow()
    {
        InitializeComponent();
        Messagebox.Show("ooooo", "text here");
    }