Hi John,
Try doing it in Microsoft 365 Admin > Settings > Org settings.
See my blog for more details:
Windows Company Branding - Microsoft 365
https://www.cloudaen.com/blogs/view/windows_company_branding_microsoft_365
If this is helpful please accept as answer or upvote.
Best regards,
Dillon Silzer, Director | Cloudaen.com | Cloudaen Computing Solutions