The project templates api is not integrated, it looks like only the api files have been auto-generated.
I would assume that an implementation exists if there already is a reference example in the Documentation (see here, select 'Java ' flavour: https://developers.asana.com/docs/instantiate-a-project-from-a-project-template)
Job result = client.projecttemplates.instantiateProject(projectTemplateGid)
.data("field", "value")
.data("field", "value")
.option("pretty", true)
.execute();