C#

private void btnAyuda_Click(object sender, EventArgs e)
{
     if (InputBox("Nueva Ventana", "Introduzca dato:", ref value) == DialogResult.OK)
     {
        //Código a ejecutar
     }
}

public static DialogResult InputBox(string title, string promptText, ref string value)
{
    Form form = new Form();
    Label label = new Label();
    TextBox textBox = new TextBox();
    Button buttonOk = new Button();
    Button buttonCancel = new Button();

    form.Text = title;
    label.Text = promptText;
    textBox.Text = value;

    buttonOk.Text = "OK";
    buttonCancel.Text = "Cancelar";
    buttonOk.DialogResult = DialogResult.OK;
    buttonCancel.DialogResult = DialogResult.Cancel;

    label.SetBounds(9, 20, 372, 13);
    textBox.SetBounds(12, 36, 372, 20);
    buttonOk.SetBounds(228, 72, 75, 23);
    buttonCancel.SetBounds(309, 72, 75, 23);

    label.AutoSize = true;
    textBox.Anchor = textBox.Anchor | AnchorStyles.Right;
    buttonOk.Anchor = AnchorStyles.Bottom | AnchorStyles.Right;
    buttonCancel.Anchor = AnchorStyles.Bottom | AnchorStyles.Right;

    form.ClientSize = new Size(396, 107);
    form.Controls.AddRange(new Control[] { label, textBox, buttonOk, buttonCancel });
    form.ClientSize = new Size(Math.Max(300, label.Right + 10), form.ClientSize.Height);
    form.FormBorderStyle = FormBorderStyle.FixedDialog;
    form.StartPosition = FormStartPosition.CenterScreen;
    form.MinimizeBox = false;
    form.MaximizeBox = false;
    form.AcceptButton = buttonOk;
    form.CancelButton = buttonCancel;

    DialogResult dialogResult = form.ShowDialog();
    value = textBox.Text;
    return dialogResult;
}