In c# textbox when enter key is pressed, how do you the index where it was pressed?

For example, I have a textbox named txtDesc and txtDesc currently contains the phrase

“The quick brown fox jumped over the lazy dog.”

My cursor position is after the word “fox” and then I pressed the enter key…

How do I know which index position I pressed the enter key?

        private void txtDesc_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter)
            {
                //enter key is down
                //How do I know which index position I pressed the enter key?
            }
        }

I should be able to get the index 19 since that’s the position after the “fox”.

I want that position so I can manually place html code “<br>” after the word “fox”. Which is where I pressed the enter key…