On iOS 9 or later, and with Touch ID enabled at the system level, Touch ID is available for use with Wickr Me.
To enable within iOS itself, go to Settings -> Touch ID & Passcode -> Add a Fingerprint
To then enable it within Wickr, go to Wickr Settings -> Auto logout. You'll need to disable any existing Auto logout time, and set to Never. This will allow you to toggle Enable Touch ID.