Set up a transient simulation.
Set power with logic dependency as shown below.
Set a goal to record the power on the same surface (or volume) that is the same as specified in Selection (below).
Now after having said that I have not got the above solution to work to my satisfaction.
My manual solution is to a) solve the transient with tabulated power that results in a temperature that is lower than the goal temp.
b) solve with tabulated power that results in a temperature higher than the goal temp.
c) Export both a) and b) to excel and interpolate new tabulated power. Rerun and manually adjust the tabulated power to achieve the goal temperature.
All this is a pain and takes days but I like the results.
Heat Flux - your first screen capture, bottom