Currently the OMV Duplicati plugin doesn't include any means to customize how the server runs and or add custom variables to the /etc/default/duplicati file that the service uses. from what I've found, this makes the default service more or less useless if you need to add any custom variables. I am not sure how hard this is to add, but having the ability to add extra options, similar to what the SSH plugin does, would be very helpful for more advanced configurations.
For Reference, this is what that file looks like by default,
# This is file is generated.
MONO_TLS_PROVIDER=legacy
DAEMON_OPTS="--webservice-port=8300 --webservice-interface=0.0.0.0"
And this is what it would look like with a single custom option added.
# This is file is generated.
MONO_TLS_PROVIDER=legacy
DAEMON_OPTS="--webservice-port=8300 --webservice-interface=0.0.0.0 --webservice-sslcertificatefile=/usr/lib/duplicati/cert/certificate.pfx"
So all edits have to be in a single line, unlike the ssh config.