I was taught that you put your hand under the mouth and with the other had you reach back to their flank and pinch & twist a little of that loose skin. They drop the bird to turn their head to see what is getting their soft underside and voila...Yeah, you can not let them have the retrieves. I think that’s a step or two down the road though. The water bottle squirt works, or you can just pinch the lips into their teeth until they open their mouth.
Whenever one of my dogs is a dink in the field I’ll take the bird and then give it right back to them and make them hold it. Repeat a couple of times then give them a drink as a reward.