I am sure that it was somewhere here: https://www.iccube.com/support/documentation/
But now it looks like it's gone.
This API will be available with icCube 6.8.1 not yet published as of the writing of this response. If you've got a pre-release dev. version than you can find it in your local icCube server:
http://localhost:8282/icCube/help/api/index.html
Hope that helps.