Hey @Huy Nguyen you could utilize custom JavaScript that parses the username and performs the expected action accordingly. You can learn more about custom javascript and b2c here : https://learn.microsoft.com/en-us/azure/active-directory-b2c/user-flow-javascript-overview
For more reference on this see the sso reference page below.
https://learn.microsoft.com/en-us/azure/active-directory-b2c/active-directory-b2c-reference-sso-custom
If you're interested in a more fleshed out solution from B2C in the portal or perhaps better documentation on how to do this, I suggest filing this as product feedback here : https://feedback.azure.com/forums/169401-azure-active-directory
And if there's enough community support the product team will look into putting this on the roadmap to implement accordingly.