you should specify temperature of the surfaces surrounding your object. If you’re simulating a spacecraft which rejects heat into space, then you should use ~3 K. But when you, for example, conduct a simulation for validation purposes, then in your model you implement a vacuum chamber in which you put the spacecraft. In this case ambient temperature depends on the temperature of the chamber’s walls, so it could be 20 C or whatever you specified in the TVAC test procedure.