After the initial step, in the procedure type of step-1, choose "Dynamic-temp-disp explicit" and then apply "load" of type thermal. see the screenshots.
Then define amplitude with magnitude and time of your time-dependent heat flux. Please note, time-period of a step and the time defined in amplitude is usually the same.
You can write a simple subroutine with DFLUX, where you can specify surfical or volumetric type heat source. Define flux(1)=F(time) as a funtion. In your case, a surf set need be defined to tell DFLUX which surface is going to act on.