There is no payment requirement for an extension.  You either owe taxes or you don't.  If you do not owe taxes then do not enter any amount.  If you owe taxes then enter the amount that you owe and the amount you will be paying with the extension request (if any).

If you do not owe taxes or are receiving a tax refund there is no reason to file an extension request.  There are no penalties for filing a tax return after the due date if no taxes are owed or filing for a tax refund.