How do I add function result to response variable as key-value pair?

I have figured out the function stack to loop through a list of records and perform simple addition of fields within each record. The last step in the loop is 'Array: Add To End' which adds the variable that is the sum of field values to the end of the loop item.

When looking at the output, it's just the value, eg. 5. How would I instead add a key to this value, eg. Total_Units: 5?