Log into your app, tap More in the lower-right corner, then tap the Settings icon in the upper-right corner. 

Tap on Fingerprint Unlock, then follow the prompts to enable.

To confirm that Fingerprint Unlock has been successfully enabled, log out of the app.  Your app login screen should now offer the Fingerprint Unlock option, or you will see the main login screen with a  Fingerprint Unlock icon in the lower-right corner (see example below).  If you are presented with the username and password fields, swipe to the side to access the Fingerprint option.