Sorry but I don’t understand what you really want to achieve. Please elaborate the following:

- “when a user signs up”. Sign up for what, for your service or for RingCentral?

- “want to have RingCentral already connected with my default account”? Do you really mean your RingCentral account or user extension? In RingCentral terms, ‘account’ means the company and ‘user extensions’ means employees.

- What does the app do with the RingCentral platform?

Thanks for prompt reply ​@PhongVu!

• “when a user signs up”: my service, my application helps to make calls directly from my application, but before making calls, they have to connect their RingCentral account first, which I don’t like, I want them to be able to make calls directly after registering in my application, for that I want to generate access and refresh tokens programmatically for them after they register, so they can use those tokens to make calls
• “want to have RingCentral already connected with my default account”: I think “RingCentral account”, because I just want to generate access and refresh tokens

The first point does not make any sense to me. Let’s say that Alex signs up for your service, and your application was authenticated by your own RingCentral account (and let’s say using your login credentials as a user in your RingCentral account). And you want them to be able to make calls, which means that you will allow Alex to make phone calls from your RingCentral phone number, using your resource?

The second point is blur and is not a requirement to generate access and refresh tokens for your customers. As you mentioned in your question that you have implemented 3-legged authentication in your app, that is enough and it always requires your customers explicitly login their RingCentral via your app (this is called authentication), to make calls from their account. Your app is just a public application that allows other RingCentral customers to operate using their own user credentials and resource.