You just have to be patient and let them make up their minds that it is okay to use.
It's possible they may never use it, though - some birds don't like baths at all.
Mine don't mind baths, but they are unclear on the bathtub concept. They won't bathe in their bathtub. They will shove their faces and bellies into their water dish and bathe there instead. And they will sit on the edge of the bathtub and drink out of it. Silly things.