Can I take free AE shuttle bus (K5) from Royal Plaza Hotel to Kowloon station in Hong Kong?

We will go HK on 5 Dec.and will stay over night at Metropark Mongkok so we will take airport bus no.A21 to the hotel .But on return trip to airport we intend to take AE at Kowloon station so we wonder that we can take AE free shuttle bus from Royal Plaza . Is it neccessary for us to buy the advanced ticket AE at airport when we arrived Hongkong or we are able to buy at Kowloon station. Thanks for your kindness