Assuming you have an outer PLINE and an inner PLINE that represents the VOID:
In the FIELD window under Field names on the left column select the Formula.
Then in the Formula window in the bottom right column right mouse click and on the cursor menu select Insert Field:

On the 2nd FIELD window that appears on top, go ahead and select the Outer PLINE object and then the AREA property and click OK:

You'll be returned to the original FIELD window and this time in the Formula box next to the first inserted value enter a (-) minus symbol to represent subtraction:

This time on the 2nd FIELD window that appears on top select the inner PLINE that represents the VOID and the AREA property and click OK:

When you're return to the original FIELD window this time click on the Evaluate button and you'll see the Preview showing the subtracted area:
Click OK and you're ready the insert this as a FIELD value. When the outer or inner (void) PLINE changes, the FIELD will update automatically after a REGEN/PLOT/ETransmit commands and etc with FIELDEVAL set to 31.