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Rotations in ROS are described using rotation quaternions, which are represented by the four orientation values you see in the pose message. A rotation quaternion must be a 4D unit vector for it to be valid, and it is non-trivial to calculate the elements yourself.

However there are useful helper classes to do this for you. The tf2::Quaternion object has a constructor which accepts angle and axis representation which is exactly what you want in your case. You have a known rotation and you know it's about the Z axis. So if you use this constructor to create a quaternion object you can then use its getX(), getY() . . getW() methods to calculate the values you need.

Hope this makes sense.