How to create a Windows Form in C# that can be seen only but can not be clicked or brought to focus

Question

I am working on an Accessibility Screen Reader application that will put a rectangle highlighter around the element that is selected on the Focused program. I am able to detect the currently focused AutomationElement, and get its BoundingRectangle property. (How is Spy++ element highlighting working?)

Given a rectangle's topLeftCorner and bottomRightCorner, I am trying to draw a highlighter rectangle using the code below:

internal class HighlighterForm : Form
{
    public HighlighterForm(Point topLeftCorner, Point bottomRightCorner, float borderWidth=5){
        int length = bottomRightCorner.X - topLeftCorner.X;
        int width = bottomRightCorner.Y - topLeftCorner.Y;
        this.Text = "Highlighter";
this.FormBorderStyle = FormBorderStyle.None;
        this.StartPosition = FormStartPosition.Manual;
        this.Location = new Point(topLeftCorner.X, topLeftCorner.Y);

        this.TopLevel = true;
        this.TopMost = true;
        this.Size = new Size(length, width);

        // Make the Form background transparent
        this.TransparencyKey = Color.Turquoise;
        this.BackColor = Color.Turquoise;

        this.Paint += (o, e) => {
            Graphics g = e.Graphics;
            using (Pen selPen = new Pen(Color.Blue, borderWidth*2))
            {
                
                g.DrawRectangle(selPen, 0, 0, length, width);
            }
        };
    }

}

And then, I am using the following snippet to create the highlighter:

Point topLeftCorner = new Point(10, 50);
Point bottomRightCorner = new Point(600, 200);
HighlighterForm highlighter = new HighlighterForm(topLeftCorner, bottomRightCorner);
highlighter.ShowDialog();

This can draw a rectangle anywhere on the screen, but the problem is that this form does not let any contents found on an app under the rectangle to be selected. For example, if the currently selected element is a page from Microsoft Word, then the user won't be able to select a paragraph because the rectangle is on top.

How can I make sure the rectangle is there only visually and cannot be interacted with or clicked on?