I dont believe that's currently possible, you can go vote for it on UserVoice: https://office365.uservoice.com/forums/289138-office-365-security-compliance/suggestions/19046497-branding-quarantine-notification-mail-and-scheduli
Branding on End-User Spam Notification

Hello everyone,
Is it possible to configure branding on End-User Spam Notification emails? Currently I have branding set but it doesn't seem to extend to the End-User Spam Notification emails.
Thanks!
2 answers
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Vasil Michev 62,156 Reputation points Microsoft MVP
2020-09-01T07:52:49.557+00:00 -
Lydia Zhou - MSFT 2,356 Reputation points Microsoft Employee
2020-09-02T02:43:46.583+00:00 How do you set branding for other messages in your organization?
Based on existing features, we cannot custom a branding template for End-User Spam Notification messages. As michev mentioned, we can post and vote this requirements in UserVoice forum.Additionally, if you just want to add your company branding in the spam notification messages, a workaround is to create a mail flow rule to append a disclaimer with your branding. You can check this article for more details, and make sure the image is available on the Internet: Format your disclaimer
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