All you have to do is create a service file at /etc/systemd/system since Debian is using systemd for service management.
Forget about copying file to /etc/init.d and /etc/defaults, they are useless for systemd.
Here is my content of /etc/systemd/system/wildfly.service
[Unit]
Description=WildFly application server
After=network.target
[Service]
Type=simple
Environment="CONFIG_FILE=standalone-full.xml"
User=wildfly
Group=wildfly
ExecStart=/opt/wildfly/bin/standalone.sh -c $CONFIG_FILE
# ExecStop=ps -ax | grep java | grep wildfly | awk {'print $1'} | xargs kill -9
[Install]
WantedBy=multi-user.target
Please note that I am using standalone-full.xml instead of standalone.xml and I was using a ExecStop script but it turns out the systemd can manage it.
I have leave it for personal note.