If you registered via Apple and opted to hide your login, your account may be associated with an email address ending in privaterelay.appleid.com. This email address was automatically generated by Apple and remains linked to your Apple ID. To view the email address associated with your account, go to the Profile page.
To log in to your Grammarly account, select Sign in with Apple at www.grammarly.com/signin or in the Grammarly applications you use.
If you want to change the email address associated with your account, you’ll need to set a password first. To do that, go to www.grammarly.com/resetpassword and enter the privaterelay.appleid.com email address associated with your Grammarly account. The password reset email will be sent to the email address associated with your Apple ID.
If you haven’t received a password reset email, see this article: I’ve requested a password reset, but I haven’t received an email yet