Hi and welcome to the Crownpeak community!
First of all, please do not use the fs-api.jar in your IDE. Despite its name it is used mostly internally to detect non-API usages. The dependency you need in your IDE is the "fs-isolated-runtime.jar". You can find it in the installation directory of FirstSpirit in "data/fslib".
If you have access to the Cloud Artifactory, it is also possible to use the dependency management of Gradle or Maven.