Create rectnagle button

GraphicsPath class provides several Methods (System.Drawing.Drawing2D)[^]. One of them is GraphicsPath.AddRectangle Method (Rectangle) (System.Drawing.Drawing2D)[^]. At the bottom of destination page, you'll find an example.

If you would like to create button with custom shape, you have to create custom class wich inherits from control. See:
Mick Doherty's .net Button Tips[^]
How to: Create a Shaped Windows Form | Microsoft Docs[^]
Walkthrough: Creating a Windows Forms Control That Takes Advantage of Visual Studio Design-Time Features | Microsoft Docs[^]
Introduction to the Line and Shape Controls (Visual Studio) | Microsoft Docs[^]
Code Magazine: Shaped .NET Windows Forms[^]

Another way is to override OnPaint method for your control.

public class TriangleButton :Button
{
    protected override void OnPaint(PaintEventArgs pevent)
    {
        Rectangle rect = this.ClientRectangle;
        using (GraphicsPath path = new GraphicsPath())
        {
			// draws triangle: /_\
			path.AddPolygon(new Point []{new Point(rect.X+rect.Width/2,rect.Y),new Point(rect.X,rect.Y+rect.Height),new Point(rect.X+rect.Width,rect.Y+rect.Height)});
			path.CloseFigure();
            this.Region = new Region(path);
            pevent.Graphics.FillPath(SystemBrushes.ButtonShadow, path); 
        }
    }
}

usage:

TriangleButton shb = new TriangleButton();
shb.Name = "ShapedButton1";
shb.Location = new Point(50, 50);
shb.Parent = this; //form

Good luck!