For rows 21-23, you knit the 9 stitches that are still on the needle and then bind them off.
Then you rejoin the yarn at the edge where you started the bind-off--at the edge of the holder where stitch 10 would have been if you'd continued all the way around.
At this point, you're casting on 12 stitches for the strap--it's knit longways--and you turn and work back across those 12 and the stitches that were on the holder. You'll have 21.
Turn and work back, making the buttonhole at the end of the strap. Knit back, turn and bind off.