First, open the overview page of any applicant in your Softgarden instance. Next, click the edit button on that applicant.
On the edit page, click on "Ursprung der Bewerbung" and then "Hinzufügen".
Copy the name out of the connection flow (do not use "Backoffice", the source name must match exactly the source in your connection flow).
Finally, enter the source into the text field and click on "+".
Now you can close the applicant page again. There is no need to save changes made to the applicant.