The Autofill feature in iOS is mostly meant to “autofill” existing credentials and not to help users register accounts. Privowny App tweaked the way it works in order to help its users register an account directly from the registration page of an app, but if the app’s developer did not purposefully implement the feature on this screen, then the key will not appear.
In that case you need to:
Open Privowny App and go to the Password Manager
Tap on thebutton at the bottom right of the page in order to open the “Add Account” screen
Fill the “Company” field
Fill the “User ID” field or generate an alias email address and copy it to the clipboard
Paste the user ID in the app’s registration form
Go back to the “Add Account” screen and fill the “Password” field or use Privowny App to generate a strong password for you and copy it to the clipboard
Tap “Save” (these credentials are now ready to be autofilled next time you need to log in)
Paste the password in the app’s registration form and finish the registration process
This way enables you to use Privowny App to generate strong and private credentials even though you couldn’t do it directly within the app’s registration’s screen.
For more on how to register accounts on your iOS device, check out the comprehensive article: How to use the Password Manager (on my Apple device)