Here is how I did it:
Difficulty: Settler
Map: Normal
2 opposing civilizations: Portugal + 1 random
Era: Information
Game progression:
Create cities until you get Casablanca, then build an airport there.
Capture Lisbon, and build an airport there if the AI hasn’t already done so.
Create and transport a worker from Casablanca to Lisbon.
I had to try several times; the worker had to leave from the Casablanca city tile and arrive directly on the Lisbon city tile, not next to it.