You can download compiled versions of the official plugins which implement this API from the plugins section of the Store control panel.
The source code for these plugins can also be found in the following GitHub repositories:
BuycraftX (Minecraft: Java Edition)
Tebex-Oxide (Rust, 7 Days To Die and Hurtworld)
Tebex-Ark (ARK: Survival Evolved)
Tebex-FiveM (FiveM / GTA V)