As Zoho Office Integrator subscription is completely based upon the API call usage. The APIKey generated will get exposed to others while integrating the API's in the Client. Hence, we have restricted the access in the Client side. Earlier, we were accepting the API requests from the Client end; however, we have restricted this flow in our latest version API's due to mentioned security reason.
As a workaround, we suggest that you integrate our API's and generate the document open url in the Server side initially. After which, you can parse the generated 'document_url' (from our JSON success response) from the Server to the Client side.
Please refer the flowchart representation in this link to know how Office Integrator works.