I have added an additional email to my Apple ID. I got an email with a code that I should enter in the email verification form. Where can I find that?

Did you check your mail account for an email from Apple, to verify the mail address?

If you didn't receive your verification or reset email - Apple Support

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Add an additional email address

1. Sign in to appleid.apple.com.
2. Select Personal Information.
3. Choose Reachable At, then select the Add button .
4. Enter your additional email address. Make sure that you're not already using it as an Apple ID. 
5. We'll send a verification to that address. Didn't get the email?
6. Enter the verification code and select Continue.

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copied from: About your Apple ID email addresses - Apple Support