code: public static BoxRetentionPolicyAssignment.Info createAssignmentToEnterprise(BoxAPIConnection api,
                                                                                   String policyID) {
          return createAssignment(api, policyID, new JsonObject().add("type", TYPE_ENTERPRISE), null);
      }
docs: Assigns retention policy with givenID to the enterprise.
  @param api the API connection to be used by the created assignment.
  @param policyID id of the assigned retention policy.
  @return info about created assignment.