You just have to use maven or gradle, you can get the API here
GitHub - LoneDev6/API-ItemsAdder: This is the ItemsAdder API repository. It serves as a Maven/Gradle reference for anyone looking to access the API without embedding/owning the entire plugin.
GitHub
<repository>
<id>jitpack-repo</id>
<url>https://jitpack.io</url>
</repository>
<dependency>
<groupId>com.github.LoneDev6</groupId>
<artifactId>api-itemsadder</artifactId>
<version>3.0.0</version>
<scope>provided</scope>
</dependency>
Javadocs
api-itemsadder 3.0.0 API
Important notes
It's very important to listen to the ItemsAdderLoadDataEvent event in order to make sure items are correctly loaded before accessing them.