Secret Chest is currently available through Apple's Test Flight. To install it, you will need to be invited to the beta. Once you have a beta invitation, open the Test Flight app and click or tap on the Install button (if you see the Open button instead then it's already installed).
Once installed, open the Secret Chest app. If the app hasn't been installed, click the New User button. If the app had previously been used by a different user, that user will be shown in the list of available accounts, given that it is possible to use two different instances of Secret Chest (e.g. home and work or for multi-user testing).
Once logged in, make sure that the Autofill extension for Secret Chest has been enabled. To do so, open System Settings (or just the Settings app on iPhone or iPad) and then search for Extensions.
From the Extensions System Setting, click or tap on Password AutoFill.
Check the box for SecretChest and then quit and re-open the Secret Chest app. Once open, it's possible to remove the banner at the top of the screen by clicking the X on it.
The first device should appear in the list of devices, provided the autofill extension has been enabled. The next step is to add another device so shards (how we break secrets up) can be installed on more than one device.