The tMap provides a graphical tool called Map Editor that allows you to create the mapping more easily from the input table to the output table using a simple drag-and-drop action. The following screenshot shows a simple mapping without any transformation from the input table to the output table.
In the screenshot above, you can see the format used to map the input column in the Expression Builder of the corresponding output column is: rowName.columnName, row1.customer_id in this example. row1 is the row identifier of the input table. It is not a constant, and it may change according to your Job. You can always read it at the top of each input table.