Now, two years later, on iOS 15.1, it's Settings > Face ID & Passwords
Enter your 6-digit passcode
Use Face ID for... and choose the app(s)
EverNote is on the list of apps.
Now go to EN > three horizontal lines bottom left > wheel top right > choose Passcode
Enter your 4-digit passcode > turn on Require passcode and, finally, Unlock with Face ID.