Principal.cs

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace R803FormularioIrregular
{
    public partial class Principal : Form
    {
        public Principal()
        {
            InitializeComponent();
        }

        private void Principal_Load(object sender, EventArgs e)
        {
            GraphicsPath path = new GraphicsPath();

            Point[] puntosA = new Point[]
            {
                new Point(0, 0),
                new Point(40, 60),
                new Point(this.Width - 100, 10)
            };

            path.AddCurve(puntosA);

            Point[] puntosB = new Point[]
            {
                new Point(this.Width - 40, this.Height - 60),
                new Point(this.Width, this.Height),
                new Point(10, this.Height)
            };
            path.AddCurve(puntosB);


            path.CloseAllFigures();

            this.Region = new Region(path);
        }
    }
}