How do I add Battle Master as an option at 3 level in my character sheet?

Only on option shows as available 

Champion is the only free fighter subclass. You either have to buy the option you want, build your character in a campaign where it is shared to you, or use homebrew to add it manually (don't publish it).

How do I buy the option? New to this so I don't know if I could build it 😳

Never mind. Found it 👍

Wait what was the answer? I have the same question

Quote from Evren6 >>

Wait what was the answer? I have the same question

 In marketplace, click a book that has the content you want and scroll down. Or just buy the whole book.