run script just before the form submits

Good day! I would like to run a certain script just before the form submits to the server. Here is what I've done so far and want to achieve in details. I created a function that will do the arithmetic and return the value inside certain field, since the value is not inputted by the user I created a function to disable them once the computation is done. The problem is when you submits form the value of disabled fields is not saved. I tried to use the fields events but this won't do the trick. Please help =( Thanks in advance Kim