How do I get SSMS to use the relative path of the current script with :r in sqlcmd mode like SSDT does?

If I have foo.sql and bar.sql in the same folder, foo.sql can reference bar.sql when run from SSDT in sqlcmd mode with :r ".\bar.sql". However, SSMS won't find it. Procmon shows SSMS is looking in %systemroot%\syswow64: How do I tell SSMS to look in the folder that the current script is saved to without explicitly declaring the path?