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There is no built-in feature of mail merge to include voter button in Outlook. You may need to use third-party tools or code to do so.
Please kindly understand that many functions are not designed at present. At present, what I can do for you is to submit feedback for your request and I have post here, and you could vote for it. If you have any suggestions or ideas, it's suggested that you could post here. Hope Microsoft will notice this in the future. Thank you for your understanding and support!
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