macro that only works on the cells I highlight

I want my macro to only work on the cells I highlight. So say columns A and B are populated with numbers and I want the sum of A and B in a new column, column C, but I only want the sum of the values I highlight. So ideally I would highlight the range, then click the button to activate the macro and It would produce the sums of the highlighted rows in column c.

Something like:

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well I'd like to be able to perform any macro/function on the selected cells whether they are in column A or Column Z. I want to be able to select anything anywhere and perform the desired macro on the selected data.