There are two good solutions, depending on whether you want simultaneous access to both the old app and your current ones. If you do, then you should run Mojave in a virtual machine from Parallels Desktop, VMware, or another virtualizer. This is quick to set up, and simplest in Parallels Desktop, which will install direct from a downloaded Mojave installer app, as listed at bit.ly/ mac371macoslegacy.
If you don’t need that degree of access, but just want to be able to start up in Mojave occasionally, then you can install a copy on an external drive, or even, if you prefer, on a volume on your internal storage.