How to get the original On-Prem AD account in SharePoint Online?

Hello

We have OnPrem AD where our users accounts are i.e.  Domain\ABC123  then, We got Azure AD, ADFS, and O365 where our users now use xxx@domain.com to connect and use SharePoint Online.

However, We now have a requirement in some custom solution in SharePoint Online to get the original On-Prem AD account value - How to do so? I mean, If the logged user is xxx@domain.com, How can I find out what is his original On-Prem AD account value e.g. Domain\xyz987

Is there a way to do so? Or, How can I sync that value into Azure AD or SharePoint Profile?