As shown in the figure, select DC (the red box in the middle of DC) in the excitation source mode on the left (the box in the leftmost toolbar).

Put this label away, double-click the dialog box and set the voltage to the required voltage (the data in the red box in the figure is set to 12V, and directly enter-12V).

I don't know how to set the time in the schematic diagram you provided. Maybe you delineated+12V and-12V, which are just net marks.

hope this helps ...