That would be for editing existing features with a survey, which sounds like what you're aiming for.
When you use mode=edit, you've got to give the survey a way to find the feature in question, so you'll need to use an additional parameter. As of a recent update, Survey123 needs to reference the globalID of the feature, not the objectID.
Stick "&mode=edit&globalId={globalid}" on the end of that URL from before, that should do it.
See this page for more info about editing existing features through a survey if you hit any speed bumps.
- Josh Carlson
Kendall County GIS