Disable "Hide My Email" in apple sign in

This is not possible. For best user experience, we recommend that developers honor users choice here

This is not possible.
Apple wil generate a random email which you can use to send a email, if needed.

There is no option for this, the user can always chooses to hide their email address.

https://developer.apple.com/documentation/authenticationservices/implementing_user_authentication_with_sign_in_with_apple

If the user is signed in at the system-level with their Apple ID, the sheet appears describing the Sign in with Apple feature, followed by another sheet allowing the user to edit the information in their account. 

The user can edit their first and last name, choose another email address as their contact information, and hide their email address from the app. 

If the user chooses to hide their email address from the app, Apple generates a proxy email address to forward email to the user’s private email address. 

Can someone explain to me how Tinder's "Login with Apple" doesn't show the option to hide the email?

Yes it does appear that Tinder is doing something different. You can not create a new account using SIWA you can only login to an existing one. If you try you will get an error saying the account does not exist and you have to create one the standard way with email/password.

I thought it was not possible to present the Existing Credentials sheet manually and it would only be displayed if the user already used SIWA in the app previously.

We'd like the same option to prevent a user from picking a proxy email address. In our case, it's because we're providing an encrypted email service, and it's not practical to give out an Apple-generated address to customers and partners.

I have seen websites like Upwork, which lets the user sign up without letting the user Hide their email. How is this possible?