Have you recorded a journal of the operation? The recorded code will give you most of what you need.
If you want to modify the recorded code to allow the user to select the plane, try searching the GTAC solution center for "select plane" or "select datum plane".
This is pretty easy to do from first principles
Dim q As Position ' Point on plane Dim n As Vector ' Unit vector normal to plane Dim p As Position ' The point we want to mirror Dim mirroredPoint As Position = p - 2*((p-q)*n)*n
The * operation is a vector dot product, where appropriate.