How to set the eGPU as the preferred GPU for programs in macOS Mojave
If you would like to set your external GPU as the preferred GPU for some of your programs on macOS 10.14 (Mojave), simply access the context menu of your program, go to "Get Info" and select the option "Prefer External GPU".
[Update December 10, 2019] Starting with Final Cut Pro 10.14.8, the external GPU can be selected in the Preferences under Playback.