$drainbias

To determine the bias added to the drain contact. It should be written in one row with three numbers which represent the start of the bias voltage the end of the bias voltage and the step size of the voltage in sequence. For example:

$drainbias 
-0.3 0.3 0.1 

This means that the bias of the gate contact will starts at 0V and increases 0.1V every step until the gate bias reached 0.4V.
i.e. -0.3 -0.2 -0.1 0 0.1 0.2 0.3

This one is similar to gate bias. If the simulation arranges different gate biases and different drain biases, it will run drain bias and then gate bias. The loop is

do vg = vgstart to vgend  by vgstep
  do vd=vdstart to vdend by vdstep
      Run
   end do
end do

The $drainbias setting in GUI interface is here
Press Contact Setting and set up the bias of the drain contact as needed!