Inset calculated value from custom dialog
I am trying to insert some value from custom dialog and have that value being added to a field's recent value.
For example : I have a field called inventory and it has 40 items right now. Now what I want is I want use to enter some value that can be added to that field's value.
I thinking to use custom dialog to take user input but custome dialog just lets me enter field value. I mean in this case what ever the user will enter will replace old field value but I want new value to be added in old value NOT replace it.
can any bod help please?