The Blackmagic TVS doesn’t have a picture in picture function like the 1 or 2 M/E units. If you want a windowed PowerPoint (or other windowed application (even YouTube video would work) then you can use the DSK with mask to simulate PiP.
Setup your secondary monitor to match the resolution of your ATEM (HDMI can be tricky, I use a DVI to SDI extender for 720p50).
In PowerPoint, select the “Slide Show” tab and Set Up Slide Show icon.
Change the Show Type to “Browsed by an individual (window)” as shown above. When you start the presentation (press ) you can resize the player and use the keyboard arrows or space bar to change slides.
Use the Down Stream Keyer in the ATEM to specify the input source for the key as your computer screen input and set the mask coordinates to block out he unwanted window border.
Just be careful not to move the mouse pointer over the PowerPoint presentation.