To use a default debtor you can enable this in the following section:
SalesorderFlow->Customer settings underneath the header Default.
You enable the switch and fill in the ISO country code and the default debtor and email address.
In the screenshot below you see that you can add more rows by using the green button "add row". It is also possible to use the red button with the white on the right in the screenshot to delete rows. 
After adding all the id's on both sides press the blue button "save mapping table". We advise you to also use the blue "save" button on the bottom to be sure that the flow saves it.