With Ubuntu 18.04, how can I have a hot corner that locks the computer?

First you need to install a GNOME shell extension called CustomCorner to be able to activate hot-corners.

Then open the extension's preferences and choose a corner, and select 'Run Command'.

In the command box, use the following

dbus-send --type=method_call --dest=org.gnome.ScreenSaver /org/gnome/ScreenSaver org.gnome.ScreenSaver.Lock

Alternatively, you can use

gnome-screensaver-command --lock

if you have the gnome-screensaver package installed.

For what it is worth:

Super+L (Super is the Windows key on PC, or the command key ⌘ on Macs) locks your screen.

I always have the screen blanker off, but it still activates as I press these keys.

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System Settings > Brightness & Lock:

Turn screen off when inactive for: Never.

Lock: ON
Lock screen after: 30 seconds.

[v] Require my password when waking from suspend.

For what it's worth:

In Kubuntu 18.04 (Ubuntu with the KDE 5 desktop), this is built in:

System Settings > Workspace - Desktop Behaviour > Screen Edges - each of the screen edges and corners can be set to various actions one of which is Lock screen.