How to put a value of a cell clicked once in to another cell using Apps Script?

[leighhobson89]

This is pretty much in the title. I want to, upon clicking a cell, run a macro to copy its value to another cell before the user can change it. This will then form a calculation of the "old value - new value" in another cell that references both values in its formula. TIA.

- The required macro code
- How to apply the macro to the cell in question.