how to avoid/stop default login prompt of TWX when we have FormLogin from Organization

If you see my previous response, The httpbasicauthenticator is responsible to authenticate the user using username and password (login prompt which you wanted to disable). There are different ootb authenticators in ThingWorx and they work on the basis of priority. The highest priority(by default 100) is of httpbasicauthenticator and as you have disabled it so you are not getting composer/login prompt while hitting the URL. Now in this case your users can only go through Form Login authentication mechanism. So when you hit the Url like /Thingworx/FormLogin/OrgName you can login through that, once login you can hit the URL /Thingworx/Composer on the same browser on a different tab and you will be able to see the composer as well. If you want only form login authentication and when user click back button of browser after you logout, they should not see login prompt you can go ahead with this approach. Please note as you have already disabled the httpbasicauthenticator, now you will not see the login prompt and only way you can get into is using form login.