@Mina Mounir Did you select the attribute under User Attribute section as well? When you signed up for a user account, did you specify a value for the custom attribute? If the attribute doesn't have a value specified, it will not be returned in the token. In that case, you should consider using a profile update policy to update the value of that attribute first. Or sign-up with a new user account for testing purpose and make sure you specify a value for the attribute and check the token that you get after subsequent sign-in for that user.
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