How can i link my secondary production account with the application which is already in production?

There are two ways to go about this:

1. Create a "PUBLIC" application (this is done while initially creating the application) which uses Authorization Code (3-Legged OAuth, and requires the use of the OAuth Redirect URL). Develop this PUBLIC application, apply for production with this application, and then you should be able to login with any of your RingCentral accounts (which are associated). This is a multi-tenant application integration and lends itself towards being published in the App Gallery at a later time if you wish, or to use publicly between your various accounts.

2. You've already done this for one of your applications, and you have the code in production already...so this should be really just a matter of time and a little effort. Login to the Developer Portal (https://developers.ringcentral.com) for each account serparately, create the Sandbox account for each of those accounts respectively, then create a private application for each account which matches the settings you have for your current production application. Execute tests in the sandbox for the new applications (just need to update the API Keys and user credentials respectively), and once 48 hours of tests have passed successfully...you should be able to apply for production, and receive API keys for the secondary production account to operate with the same code base (I would recommend separate deployed instances configured for each account if you go this route, since the app is not configured for multi-tenancy in this use case).

Thank you for your reply. I want to go with the second use case mentioned by you. Yes, I've done this for one of my applications.The process I followed is :

I've created a sandbox account with my Primary (Main) account that is why now my application (in production) has the account credentials of my Primary account.

Can you please tell me that is there any other way to associate my application (in production) with Secondary account rather than creating new sandbox accounts with each of my Secondary account and following the whole process from start? It would take a lot of time.

Currently there is not, but I will create a ticket for our platform and engineering team to evaluate this feature for development.

Can you please tell the process of creating separate sandbox accounts for each of my secondary accounts? I can't see any option for creating sandbox account after logging in to Developer Portal (https://developers.ringcentral.com) for each secondary account.

To create Sandbox accounts in your secondary RingCentral Accounts, you will need:

1. Login to the Developer Portal using an Admin-User from your secondary account. If you are unable to access the Developer Portal using these credentials from your secondary account, please contact your Account Executive to ensure this secondary RingCentral account qualifies for access to the Connect Platform.

2. Once you have authenticated into the Developer Portal using the secondary account Admin-User credentials, hover over your name in the top-right of the UI and select the "Sandbox Accounts" navigation item. It is here that you will define the Sandbox account for your secondary RingCentral Account. You can see this navigation element highlighted in the screenshot below.