Every Discord account has a permanent numeric ID, usually 17–20 digits long. It stays the same even if the person changes their username, display name, or avatar. That is why staff ask for IDs instead of names when you report someone.
With Developer Mode enabled, right-click the person's avatar or name on desktop and choose Copy User ID. On mobile, open their profile, tap the menu (⋯), and tap Copy ID.
Paste the ID into your report or ticket. If you do not see the copy option, enable Developer Mode first — our guide on that walks you through it step by step.