Hi
By default when you first activate a O365's plan you need to log in with your onmicrosoft's account.
You can add frungalis.se's domain inside your tenant to be able to signin with that domain, but you have verification to do with Microsoft and your registrar to make that work.
See that link for more information; https://learn.microsoft.com/en-us/microsoft-365/admin/setup/add-domain?view=o365-worldwide