Iirc, ERAS automatically imports your legal name from your AAMC account (which specifically requires legal name). That name also has to match what's in your USMLE records in order to import your test scores.
edit: not sure whether you're thinking about changing your name, but one piece to consider is that although it may be too late for ERAS, if you really don't want your current legal name to be out there and associated with you professionally, it is preferable to change it sooner rather than later. Once you have a medical license, NPI number, etc. under a certain name, that information is publicly available and remains easy for people to find once you've changed your name.