ACHIEVEMENTGET(<text>,[itemid[:damage]])
Displays an "Advancement made!" popup you get when completing an advancement with a custom message and optionally a custom item.
The successor is TOAST with the option to create other types of popups.
Example
// With no item id; Shows an grass block
ACHIEVEMENTGET("Hello Grass")
// With an item id
ACHIEVEMENTGET("Hello Stone","stone")
// With an item id and metadata
ACHIEVEMENTGET("Hello Birch","log:2")