I found your question while searching for a solution myself, and can contribute the following:
gives you a list of parameters for the Nexcloud Client binary call.
--background seems to be one of them.
In your KDE environment, open the Autostart control panel view and pick the Nextcloud Client from the list. Open the properties window and go to the program tab. You should see the command here. Just extend it like this:
Tested on Kubuntu 19.10.