You can also buy the pass from the DB ticket machine at any S-Bahn station, or from almost all MVV (city transport) ticket machines at U-Bahn, S-Bahn, or tram stops. So if you're taking public transport to Hbf within the pass's validity period (after 9 am if a weekday, or any time if a Sat. or Sun.) that ride will be covered, too.

ETA: As you probably know, you can also buy the pass for a future date, too (at any of the sales points), but I wrote the above assuming that you plan to buy on the day of travel.