Do you mean the option to take photos while recording a video using the Camera app? This option should be working on your device during the Beta Programme . If it does not work for you, please report the problem via the Samsung Members application.
Have you tried to open the Edge Panel - > Settings icon - > Smart Select? There are many interesting functions, such as 'Rectangle', 'Oval', 'Animation', and 'Pin to screen'. Maybe one of them is the option you need?