To attach a flat-backed finding

The strongest bond would come from using a drop or two of resin. You can also place the finding into the resin as it sets up. If you don't want it to sink down into your resin, you can wait for about 1 hour for it to thicken up first, testing the thickness with your mixing wand or a pin.

If you're not mixing up a second batch of resin, you can use a strong adhesive such as E6000 (although it's stinky), or ZapAGap, a great super-glue.