How to make a corded button loop
Cut a length of fine piping or blind cord at least twice the length of the loop. Fold the loop piece in half lengthways, right sides together, with the cord inside. Pull the cord halfway out of the top, then machine stitch across the top of the loop fabric using a zipper foot, sewing to the centre of the cord.
Turn and stitch down the length of the loop fabric close to the cord. Secure the stitches at the end, but do not close it. Pull the end of the cord out of the open end of the loop until it turns inside out. The loop is now right side out with the cord inside. Cut off any surplus.