I used a paperclip to raise the straight stitches for the base threads.
Then I made buttonhole stitches on these base threads.
I secured the thread at the backside after removing the paperclip.
Then I stitched more French knots below the buttonhole bar. I used a pin to hold the bar and make it easy to stitch.
And this is the result.
I stitched this raised buttonhole bar with a cotton perle 8.