In Boyd Genie, you can add devices to your heat sink before simulating thermal performance. Boyd Genie has a graphical user interface (GUI) for heat source placement on your heat sink, but an important thing to consider is how your heat sink is oriented in relation to the GUI.
If you take a heat sink resting on your desk with the base resting on the surface, pick it up, make sure the fins are oriented vertically and press it to the screen where the GUI is, that’s how your heat sources and heat sink are placed in relation to each other.