If you used Sign in with Apple, Apple may have created a hidden address ending in @privaterelay.appleid.com. That relay address can be the email linked to your Chatex account even if you normally think of another inbox as your Apple email.
On iPhone or iPad
Open Settings, then tap your name.
Tap Sign in with Apple.
Select Chatex from the app list. If you chose Hide My Email, Apple shows the @privaterelay.appleid.com address there.
On Mac
Open the Apple menu, then choose System Settings.
Click your name, then click Sign in with Apple.
Select Chatex to see whether Apple assigned a relay email for that account.
If you do not see it
You may not have used Sign in with Apple for that Chatex account.
Try your normal Apple sign-in again before creating another account.
If you are recovering billing access, include the relay email in the support flow together with any support code or charge details you have.
Apple’s official instructions are here: How to use Hide My Email with Sign in with Apple.