Here’s how to do it:

Log into the CLI of your Ubuntu 18.04 server.
sudo su
sudo nano /etc/environment

Now add in your proxy variables in the following format:

http_proxy="http://my.proxyserver.net:8080/"
https_proxy="http://my.proxyserver.net:8080/"
ftp_proxy="http://my.proxyserver.net:8080/"
no_proxy="localhost,127.0.0.1,::1

Disconnect from CLI and login again for /etc/environment to take affect.