How do I add custom Fighting Style to Subclasses

[wheelercub]

I've been working on the Mystic class in HL and am nearly finished. Unfortunately one thing has me stumped. I can't figure out how to allow a class variant to select a Fighting Style, similar to a Ranger or Fighter. The variant Mystic is called the Order of the Knife, and he should be able to choose between 3 different fighting styles once the variant is selected.

I've tried copying other content, but it never lets me select the styles. Choices should be Great Weapon fighting, Two-weapon fighting, or Single Weapon Fighting. I want to be able to have my damage stats updated accordingly based on the Fighting style, as well as being able to select one of the 3 mentioned above.

Any help will be welcome.