Each CEX.IO account has a unique UserID/Username. One may need the UserID when logging in, setting up an API, or when communicating with support. Users can easily find it.
On the web:
Log in to your account and hover the mouse over your name in the top right corner and choose Social networks from the dropdown list.
The screenshot given below shows the location of the UserID on the above-mentioned link:
On the mobile app:
Log in to your account and tap the Profile menu. You will find your User ID on the top of the screen.