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 uses once logged in.

In Rambox, you can register this URL by getting logged in the app, then right-clicking to get the context menu, and selecting "Set current URL as app's default".

Take into 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 it retains the login credentials there.