This usually happens when the custom app's URL points to the login form or the homepage, instead of pointing to the URL the app use once logged. In Rambox you can register this URL, by getting logged in the app, then right-click to get the context menu and selecting "Set current URL as app's default".

Take account that not all apps store credentials in browsers, so please check the behavior of your custom app in your browser (Chrome, Firefox, etc.) to check if in it retains the login credentials.