Nope, and you can’t. Incognito mode is designed to leave no traces of the browsing activity that occurred on the device. Once a tab is closed, you need to manually re-open it by finding the same link. Discord is spyware because it collects all information that passes through its communication platform.
The easiest way to report someone on Discord is to simply tell a moderator of the server that the inappropriate behavior occurred in. If you want to report something to Discord’s own Trust & Safety team, you’ll need to turn on the app’s Developer Mode.