The ™ code would only work in HTML, so that should work in your thank-you page customization fields.
For anything that would appear in email, we only send messages in plaintext to ensure the widest possible compatibility for buyer recipients, but we do use the UTF-8 character set encoding for emails, which should respect special characters such as â„¢, so you should be able to just copy that symbol and paste it into your email message or template wherever you wish.