If you're unable to find the Submit to Turnitin option when attempting to upload from a third-party app, it's likely that you need to enable it.
1. Navigate to the share/send copy area of the third-party app from which you are submitting.
2. Found to the right of the list of apps that you may share your document to, tap the More (...) icon.
3. From the Activities box, tap the Submit to Turnitin toggle, switching it to green, then select Done from the top right-hand corner.
3. Tap the Submit to Turnitin icon to proceed with your submission.