This is how it'll look like in the end:

STEP 1 - CREATE TASK AND DIALOG

• Create a new Task and call it Fingerprint
• Add an AutoTools Dialog action
• Select the Fingerprint dialog type
• Set the Title to Authentication
• Set the Text to Please use your fingerprint to continue

In order for the task to work when a non-recognized fingerprint is detected you need to enable the Continue Task After Error option in the Tasker config screen for this action. Otherwise the task will just stop if the wrong fingerprint it used.

STEP 2 - DO STUFF DEPENDING ON RESULT

• Add an If action in Tasker
• Check if %atbutton matches 2

%atbutton will contain the button you pressed. 2 is for the fingerprint "button", 1 is for the Cancel button in the dialog

• Add Else and End If actions
• In the If section add a Flash action with the text Recognized! You can continue!
• In the Else section add a Flash action with the text Denied! App will now close!

STEP 3 - GO HOME IF DENIED

• In the Else section add a Go Home action so that the app closes if the fingerprint is not recognized

STEP 4 - APPLY PROTECTION TO APPS

• Create a new profile and use the Application condition
• Choose the apps you want to protect
• Go back and choose the Fingerprint task as the entry task
• Back out of Tasker to save

STEP 5 - TEST

If you now open the calculator app your fingerprint will be required to operate it!