How do I add an app or account to Spanbox after installing it?

Once it's open, there's an add/plus control to pick an app from the list of 60+, choose one (say WhatsApp) and it opens that app's normal login inside Spanbox, scan the QR or sign in as usual and it's connected. To add a second account of the same app, just add the app again as a new entry; each one gets its own isolated session, so you can name them 'WhatsApp Personal' and 'WhatsApp Work'. Repeat for your other apps up to your plan's limit (3 on free). After that they all show up in the sidebar and the unified inbox, and ⌘K jumps you to any of them.