Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Based on what I know, I do not think you are able to make that change.
Outlook.com is the domain used by Microsoft's consumer email system.
yyyyy.onmicrosoft.com is the administrative domain when you first setup a Microsoft365 business account.
You should buy a personalized domain and use that instead (eg kamalthgroups.com)
The commercial versions of Microsoft365 allows personalized domains (eg vc @ kamalthgroups.com) if you are able to get the domain, if not a variation of it. This is the preferred way for a business domain as you can add more people to the domain as the company grows. Eg devi @ kamalthgroups.com, trisha @ kamathgroups.com etc
The outlook.com domain does not give you the flexibility.