I had some trouble looking into details on how to start these missions and wanted to share what I found for others who are just starting the game. These missions will show up as circles on the map above the main map. They do not start to show up until you complete the Capital Building stronghold, but some will not show up until you get to World Tier 5. To get to WT5 you need to complete 6 missions and 3 strongholds again as invaded missions. This is just the same mission again with slightly different objectives and new enemies.
You also need to increase you gear score as you complete these missions. This is pretty easy as you can just use the gear you get during the 3 missions for each tier to level up.
Once you reach WT5, this mission will show up on the map. It's a bit longer than a normal mission (or feels like it is at least), but is still pretty straight forward to complete.