Please open your __APP__ domain: https://__DOMAIN____PATH__

The username is the administrator's email address
The password is the administrator one you filled during the installation
The secret is: __API_SECRET__

Please note that if you did NOT install the application in public mode, you should go to the YunoHost login portal first to authenticate yourself in order to access to the application.

If the application only shows a blank page instead of the login screen, you can try turning off 'Enhanced Tracking Protection' in Firefox. See https://github.com/YunoHost-Apps/invoiceninja5_ynh/issues/184