How do I download Mojave when it doesn't appear in app store?

Go to the support article How to download and install macOS - Apple Support

Some links require you go through the Mac App Store and some older OS versions have a direct download link

This resource is on an official Apple page too, so in general it is a safer and more reliable resource since I see the Mojave URL has apparently already changed several times (including not being the one just provided).

MacBook Pro 15″, macOS 10.14

How do I download Mojave

Currently operating 10.13.6 on MacBook Pro (15-inch, 2018)

Check for sure which macOS version -> go to Apple menu  -> About This Mac.

You can update your MacBook Pro (15-inch, 2018) to macOS Catalina - macOS 10.15.7, see -> macOS Catalina is compatible with these computers -> macOS Catalina is compatible with these computers - Apple Support

How to update?, follow the instructions in -> Related Article: Update macOS on Mac

You can also check if your Mac is eligible for an update, you'll see Software Update in the Dock.

Try this link:

https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12

Yep, both links opens the same page in the Mac App Store. Nothing fishy going on here.

Just general advice to users. ;-)