Using New Outlook on Windows for personal email, calendar, and contact management
Use INS + F5 to get a list of form field controls on the page. There are 69 fields. The fastest way to get to the Subject edit field is to then press the letter "S" until you hear "subject edit" and then press ENTER. That will place the cursor in the Subject field.
Alternatively, you can TAB past the address fields, without filling in an address and go directly to the Subject field. Type your subject and tab to the message body and type your message. Then SHIFT + TAB back to the TO field and fill in the address(es).
That's the two workarounds that I've found.