With the longies I crocheted, I just crocheted a chain the length I needed the drawstring, then used my hook to weave/thread it around the waist. You could probably use a yarn needle, also.

HTH

 

I usually make a casing by turning the top edge down 1", then stitching close to the edge almost all the way around for elastic, and all the way around for a drawstring. I cut 1.5 inch elastic to waist measurement minus 1 inch and thread it through the casing, sew the ends together and then finish the casing stitching. For a drawstring, I crochet two strands together to form a single stitch chain and knot the ends. I then take a seam ripper and open up the center front seam above the casing stitching line, then thread the drawstring through. To thread anything through a casing, I put a safety pin on the end and skootch it through.

You could try threading the drawstring in and out at the waistline like knitted and crocheted longies, but the knit would need to be pretty loose and you would have to finish the top edge somehow if it is a cut edge, like by serging.

HTH!