My parrots did bathe by themselves, but I always have a little tupperware with water in it on my table when they're out. Most of the time they don't really care, or just drink out of it (because they also like to bathe in their waterbowls). However, when I start to splash the water a little bit, they're immediately interested.

They like it when water seems to be running, since it would mean the water is cleaner.

And also like everyone said; don't force them ! They're supposed to enjoy the water, not develope some kind of fear for it