This can happen and it will depend on the App you're using or website you are visiting. If that App or website doesn't pass a good hint to Strongbox or the iOS AutoFill subsystem it just won't work.
You can find out what "Hint" that App or Safari is passing to Strongbox by
- Open Strongbox in AutoFill Mode
- This can be done simply by opening Safari for example.
- Go to a website with a login (e.g. https://accounts.google.com)
- Tap on the username field and you will see a QuickType suggestion or just "Passwords" above your keyboard.
- If you see Passwords just tap that and select "Strongbox".
- If however there is a QuickType suggestion there, then tap the little key icon to the right, and select "Strongbox..."
- Unlock your database of choice.
- Clear / Cancel out of the Search field to reveal the "Preferences" button in the bottom left.
- Tap Preferences (Gear icon in the bottom left)
- Turn on "Add Service IDs to Notes"
- Tap Done
- On the main AutoFill screen now, Select your database and Unlock it
- Cancel out of Search
- Tap the + button in the bottom right
Strongbox will prepare a new entry. In the Notes field you'll see the hint or 'Service ID' that is passed to Strongbox.
If it is there, you can use this URL in the entry and it will show up as the bottom of your screen. If it is not there, the 3rd party app or website is not integrating correctly with the iOS AutoFill system.