I am unsure if this requires a native embassy built, or if it requires that you trigger the trade monopoly victory. I did both to recreate conditions for unlocking it. Please feedback with what worked for you.
I played a skirmish game, 3 players, on Honshu. I picked three so that neither would start too close to the native villages for me to build trading posts on them.
I immediately used the cheat a whole lot of love several times to give me all the resources needed, and sent my explorer to build trade posts on the villages. I set villagers to build a lot of houses, and used cheat speed always wins to make all building/research instant.
Be careful with this cheat, as it works for the AI too. Quickly max out your age so you can build the Capitol and research spies, allowing you to keep an eye on the enemy.
Send some villagers to the other landmass to build a trade post on the trade route.
When you have trade posts on all three, spam cheat tuck tuck tuck to gain a bunch of monster trucks and roll over all enemy buildings and units. You may need to build boats to take out any they have made.
If this doesn't work, try starting a trade monopoly victory. Let me know what works, be constructive with your criticism.