You need to draw a square to the left of the text (square Panel element) 3 black borders 1 white on the right background - the background is white and this white border will overlap the left black line in the TextBox element

private void panel1_Paint(object sender, PaintEventArgs e)
{
Graphics g1 = e.Graphics;
Panel p = (Panel)sender;
g1.DrawLine(new Pen(Color.Black, 1), 0, 0, p.Width, 0);
g1.DrawLine(new Pen(Color.Black, 1), 0, 0, 0, p.Height);
g1.DrawLine(new Pen(Color.Black, 1), 0, p.Height-1, p.Width, p.Height-1);
}