This is only available on BenQ Boards with EDLA License (RP04 and RM04). Annotation is also only available in the Browser, not the Google Slides application.
1) Open your Chrome Browser and make sure that your browser is running in Desktop Mode. Click 
2) Confirm that Desktop Site is checked. If not, please check Desktop Site.
3) Open up a new tab and browse to https://www.google.com so you can access your Google Slides apps via the App Launcher 
.
4) Press and hold Slides and select Open in new tab.
5) With Slides opened in the browser, you can open a Google Slides > Slideshow
6) Bottom left of your presentation, tap and click Turn on the pen.
Please note that any annotation you do during this presentation won't be saved to the Slide and will only exist during this presentation.