code: public BoxRequestsFile.AddFileToCollection getAddToCollectionRequest(String fileId, String collectionId) {
          BoxRequestsFile.AddFileToCollection request = new BoxRequestsFile.AddFileToCollection(fileId, collectionId, getFileInfoUrl(fileId), mSession);
          return request;
      }
docs: Gets a request that adds a file to a collection
  
  @param fileId        id of file to add to collection
  @param collectionId    id of collection to add the file to
  @return      request to add a file to a collection