I recently created an Ubuntu 18.04 on Azure to run a test service I work on in my spare time and noticed that the "/etc/rc.local" file doesn't exist by default anymore. There are other (probably preferred) ways to start applications when the server boots by rc.local was in my opinion, the absolute easiest.
TL;DR => "etc/rc.local" still works, you just have to create it and then give it execute permissions. Remember that it is a shell script so it will need to be setup as one.
#!/bin/sh -e cd /home/admin/test-service ./start-service & sleep 10 # Always leave this line here exit 0
sudo chmod 755 /etc/rc.local