How to rotate a bone about a certain axis in C++

I am rotating bones using C++ code and an animation blueprint that uses custom variables. I want to be able to write as the functionality of those custom variables a rotation about a specific axis (z).

This is how I am using these variables now, though they don’t really have any functionality because I just copied this from a tutorial. So given this (this is my actor class cpp), how can I define these variables to rotate around a specific axis?

Thank you for your time.