The "official" method is to unscrew the barrel and pour water into it. As the opening is so small, it sometimes gets a bubble that blocks it; a slight squeeze should move that and continue to fill.
Another method that might work better is to squeeze the empty tube and submerge the opening in a glass of water, then release. You can see this in this video at about 4:45
https://www.youtube.com/watch?v=Xk9X-7wq2s0