After you are in a Zoom meeting:
Start PowerPoint
Open Presentation
Click SLIDE SHOW menu item
Click FROM BEGINNING ribbon item
ALT-TAB back to Zoom window
Click SHARE SCREEN button in Zoom
Select SLIDE SHOW Screen from available share items
If you have audio, check SHARE COMPUTER SOUND box
Click blue SHARE Button when time to share
Advance slides with click, spacebar or arrow keys
At end STOP SHARE SCREEN (Host may do this for you)