That looks good. I edited it to code tags instead of quote to make it display properly.
So, if I understand correctly, the connection works but you have to specify a username to make it work in Duplicati? I think there was a post about this and the suggestion was to just put in a dummy username and leave the password empty. Try to see if that works.
It seems that the Webdav backend implementation explicitly supports having an empty username to disable authentication, but this is not reflected in the UI. Otherwise, a trick you could do to bypass the UI verification is exporting the configuration to JSON after you are finished. Edit the destination URL to remove the username, then import it with Save immediately. That will make it difficult to change the job, though.