How to find the AD user that logged in to SQL Server using SQL Login
It's not possible I'm afraid.  This is one of the down sides of using SQL Server logins.  The only thing you can do is to look at the Hostname.  Most people connect using their local machine and if your company has a list of user to machine name then you can track them that way.

Unfortunately this information is not normally logged.  You will have to set up a trace, extended events session or server audit and collect login events and make sure you include the hostname property.