The welcome screen isn't in my menu.Hi Eddie, and welcome to Linux.org. I use Mint, which is an ubuntu derivative. The Welcome screen is an actual Menu item in my Cinnamon. Just set it to show on boot, but realize if you are using ubuntu on a live boot any changes you make to the default won't become permanent unless you use a program like "Persistence" on your boot disk or drive.
Hi Eddie, and welcome to the forums! As @wizardfromoz noted, it is a "first-run only" screen. I just installed Ubuntu 18.04 Gnome, the same as you have, and the "Welcome screen" is actually a series of screens highlighting some of the new things in 18.04, prompting you to setup Livepatch, asking permission to send telemetry data back to Canonical (allows by default), and lastly providing a button to "Open software now" (needs working internet connection).The welcome screen isn't in my menu.