Bathroom sink drain sieve
Is there a drain lever above it, covering the upper whole in the tub with screws in it? If so cover the bottom drain with a towel, first so the screws don't fall down into it..
Next remove the screws and lever (screw) as well, then pull out the whole mechanism,you should see a spring attached to the bottom of it. (That is what the stopper and rocking bar attach to.)
Anyway, this should then allow you to remove the stopper,( Can't you just pull it out?.. There should be a long bar attached to the stopper,( that is the rocking bar) You then simply put the mechanism back in place and reinsert the screws, you must be cautious and make sure you have the drain strainer in place at all times, otherwise you will have a large whole there any then anything can fall in there...Good Luck...
Read the above post, from (Me) woodpecker, about doing the happy dance, and you will see what can happen when the stopper is removed....as I also removed the stopper you are talking about...YOU MUST MAKE SURE YOU HAVE THE STRAINER FIRST, and insert it. or you may be sorry,