Google Slides does not provide the needed context for the Co:Writer Universal Extension to work properly. If you are on a Mac or PC we recommend using the Co:Writer Universal desktop application.
The Co:Writer Universal Desktop application gets partial context from Google Slides. Google Slides provides forward context, but does not provide backward context or cursor location.
For Chromebooks we recommend using the Co:Writer Universal Chrome app to type out the text, then use copy/paste to enter the text into Google Slides.