We have two CSV files. The first has with SKU and LOCATION headers:

SKU, LOCATION
FS1, Chicago      
FS2, Philadelphia

The second file has the headers SKU and AMOUNT.

SKU, AMOUNT
FS1, 100      
FS2, 200

We want a third file with SKU, LOCATION and AMOUNT.

SKU, LOCATION, AMOUNT
FS1, Chicago, 100
FS2, Philadelphia, 200

Approach

Load the first CSV file. Click on Selected Join Key and select SKU as key field.

Select the SKU and LOCATION headers.

Load the second CSV file. The Selected Join Key will have a green check mark next to it indicating that both files have the SKU header.

Select the AMOUNT header.

Click the PROCESS button. The result can now be saved.