You can do both.

To add stabilizer (CYA), you can either put it in an old sock and hang it in front of a return jet and give it a squeeze once in awhile to help it dissolve faster or pour it slowly into the skimmer and let it dissolve in the filter. (That is what I do.) If you do put it into the skimmer, keep your pump running 24/7 for the next 4 or 5 days. Don't rerun the CYA test for a week after you add it so you will make sure it is dissolved. Don't want to waste your testing reagents.