Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hello
Unfortunately no, this is by design.
The definition of "organization" account is not according to how you use it. You can use it for personal purposes, but it can still be recognized by Msft as an organization account. It is based on who provides the email address/domain. If it is from Microsoft, custom domain service from Microsoft is all considered a commercial product. Or, if it is from a reseller/vendor that's using Microsoft's service/in infrastructure, it can be still being seen as a commercial account/organization account.
If you just need to add this email address so that you can get the 2FA code to this email address, you don't need to add it as alias. Alias serves as username only, nothing more. So, if you want to receive 2FA code, go to https://account.live.com > Security > Manage how I sign in. Choose Email a code, and you should be able to use your custom domain email address here.