how to add data automatically

i create simple inventory application that consists many items. applicant is able to request for item

After applicant click request the request form will be displayed

as we can see there are 2 quantities , the first one is current item quantity and second one is quantity that requested by applicant. so the question is , how to make quantity in data list automatically update after applicant submit the form? please help me thank you 

below is the data list