I like to use a crochet cast on because it matches the look of the bindoff. So the top & bottom of the buttonhole look the same.
You bind off 4 stitches. The on the row back, crochet on 4 stitches. Use a crochet hook the same mm size as your needles.
This is also the way I make a provisional cast on. Easier for me that the crochet chain with knitting in the bumps.
Here is a link with this type of cast on.