The list box displays the keys mapped if any.
Press the key to map. Select the keyboard icon button to bring up the soft keyboard if needed. Once a key is pressed and confirmed to map, a new row is added. Select the Key not mapped entry to map the key to a function or macro. If a macro is selected, select the Detail disclosure button to define the actual macro.
Select the Edit option to remove a key mapping.
Please note that prior iOS 13.4 various keys on the external keyboard (like the function keys F1, F2...) are not supported by iOS and cannot be mapped.