I think you should first get around to know the system before you start using it in an non-orthodox way. Coding behavior trees directly in C++ is not advised - that’s why we created the editor, because coding it would be too much of a hassle.
However, if you insist od creating BTs in code looks at BTTest.cpp, it’s where some of out BT unittests live - this should get you going, but I believe you’ll have hard time doing anything non-trivial.
Cheers,
–mieszko