Windows Forms Textbox Enter key

I have a TextBox, but I can't find any source explaining how to call a function when a button is pressed down.

public Simple()
{
    Text = "Server Command Line";
    Size = new Size(800, 400);

    CenterToScreen();

    Button button = new Button();
    TextBox txt = new TextBox ();

    txt.Location = new Point (20, Size.Height - 70);
    txt.Size = new Size (600, 30);
    txt.Parent = this;

    button.Text = "SEND";
    button.Size = new Size (50, 20);
    button.Location = new Point(620, Size.Height-70);
    button.Parent = this;
    button.Click += new EventHandler(Submit);
}

Some sources tell me to use a function, but I don't understand how it is going to get called.