With supporting SimpleBuildStep introduced in Jenkins 1.577...
(copied from https://github.com/jenkinsci/copyartifact-plugin/pull/43)
Atm, it works using the base 'step' step (i.e. you need to specify the $class) e.g.
step($class: 'hudson.plugins.copyartifact.CopyArtifact', projectName: 'project1', filter: 'hellothere.txt')
Once the workflow apis are published we can build a dedicated step for this, it would look something like the following:
copy(projectName: 'project1', filter: 'hellothere.txt')