Implementing Captcha in the new Dynamics Portal in comparison the the old ADX portal is an absolute cakewalk. Here’s what you do:
Find the Entity Form or Web Form Step that you want to enable Captcha on, under the “Form Options” tab of the form set “Enable Captcha”
Err… that’s it! Although note, the Captcha looks rubbish and you’ll probably want to move it’s location, wiggle it around by targeting the class
captcha-cell in your css / JS.