To start Boralis you would need to find Chromie and speak to her.

    1. Look for a hour glass on the Stormwind map; (you will need to be in Stormwind.)
    2. Speak to Chromie and she will offer expansions, hopefully from that screen you can select Battle for Azeroth.
    3. You an ignore or abandon the quest she gives.

    If the guides didnt detect Chromie time, open up the main window and in the search field type in Chromie Time and click on the guide.
    You could just load up the Battle for Azeroth and load up the intro guide if you haven’t done that on another character before.

    I hope that helps and that you enjoy the BFA storyline. All the best!