When you sign up with "Sign in with Apple," Apple may create a private relay email address (something like xxxxx@privaterelay.appleid.com) instead of using your real email. That relay email is what's on your Split Pay account, and you'll need it any time you contact support.
The fastest way to find it is to log into Split Pay with Apple and check your account settings — your email shows up there.
If you can't log into Split Pay, you can find it through Apple directly:
Go to appleid.apple.com and sign in
Under your name, choose "Sign-In and Security"
Choose "Sign in with Apple"
Choose "Split Pay" from the app list
Under "Hide My Email," you'll see your relay email
Save it somewhere you can find it later — Apple's relay email is a privacy feature, but it means the email on your Split Pay account looks random instead of your actual email. Anything we send still gets forwarded to your real inbox.