Please follow the below steps to add the PostMessage domains.

1. Go to Office Integrator Dashboard and click on 'White Labelling' option in the left corner.

2. In the next page, select 'Security Controls' option.

3. Then, enable the 'PostMessage Domains' option; enter your PostMessage domain which needs to be added and click on 'Add' button as shown in the image.

4. Your PostMessage allowed domain will be successfully added. 

To know more about PostMessage API, please refer this link.

List of available PostMessage API 

Zoho Writer - https://www.zoho.com/officeintegrator/api/v1/postmessage-api-list.html 

Zoho Sheet -  https://www.zoho.com/officeintegrator/api/v1/sheet-postmessage-api-list.html 

Zoho Show - https://www.zoho.com/officeintegrator/api/v1/show-postmessage-api-list.html