Welcome to the RingCentral Community

I think you can do that if you implement the authentication using JWT token in the backend and provide a secure way to get the valid tokens from the backend whenever you need to call the platform APIs.

To do that, remove the login function from the client side, and provide a method to get the tokens (you will have to handle multiple users to make sure that your backend returns the right tokens to the right user). Once you have a valid tokens, set the tokens to the platform object and call any API. Here is my untested example. /getToken is your server method that login and return the tokens object.

function getTokens(userId) {
    var getting = $.get( `getToken?userId=userId` );
    getting.done(function( res ) {
      if (res.status == "ok") {
          platform = sdk.platform();
                        await platform.auth().setData(res.tokens)
                        // Call platform API
      }else if (res.status == "error"){
        // handle error
      }
    });
}