For the people who don't want an extra hanging off their machine (I personally don't care, I might try the binder clip or the spool with eyelet, but for travelling machines it might be inconvenient) there's a spool pin you can get that has a thread hole drilled into it for exactly this purpose.
Sites I've seen online show it as being for the 222, so it should work for the 221 as well.
Part number 12403, the prices I've seen online are a little steep. Cdn cost on the pin is under $1, so paying $4 is a little out of line, so if you do look for one, check a few places.