Hello,
Thank you for reaching out to the community, just to inform you Microsoft 365 family account no longer supports customer domain unlike the business and enterprise subscription. However if you would like to have a customer domain you must upgrade to Microsoft 365 business plan. Just sign in to Microsoft 365 admin center, navigate to settings and select domains. Now select add domains and follow the step to verify the ownership, go to Users and select Active users to edit your user name and select the new domain.
Another option is to create a new alias using another Microsoft supported domain like outlook.com, outlook.ph etc., here is how go to https://account.microsoft.com and choose Info. Now select Manage how you sign in to Microsoft, click Add email and Create or add another email address you can even set it as your Primary alias as an option.
I hope this answer is helpful if you have further questions feel free to reply back
Regards,
Marcelo