how to add a check box in form

i am adding the check box to calculating the net amount in the purchase order line level.

my requirement is 

case1: i want to add the total excise amount to net amount before calculating the discount 

case2: i want to add the total excise amount after calculating the discount in the Purchase order line details.

In this requirement i am adding one check box in the form before the discount amount.

i want to write a code to check and un check the checkbox in the form.