I am working on an automation stuff in macOS Sierra (10.12.2). By using python\'s atomac support I can launch the safari browser and make the settings enabled via Safari -> Pre
I found the answer from: https://developer.apple.com/documentation/webkit/testing_with_webdriver_in_safari
safaridriver --enable
It will prompt for an admin password, so some people setup passwordless sudo (dangerous) or do another workaround like calling from an Applescript
I tested that this works with Mojave and Safari 12.0.3 and verified that it works regardless of if the Develop menu is enabled, though you probably want to save yourself the clicks and just enable that too:
defaults write com.apple.Safari IncludeDevelopMenu 1