Not sure, i looked it up online and build one in both my two good settlements and assigned a settler to it.
If you want to play this game with the settlements, i'd recommand to get 6 Charisma as early as you can manage, and get 1 level of "local leader".
These populations take a while to crowd, and to build the good basics, you need to be able to share your workstation, wich you do via traderoutes.
Just scrap everything in sanctuary, wall it off and place drones, build a crop field and waterpumps(as the game says) and beds. Build a generator and connect with a radio tower.
I dislike the waterpumps, so i build a watercleaning thingie and put a few pylons over the wall to connect power to it. Just assign settlers to crops to get enough food.
Here is the tricky thing. A traderoute means you take a settler to do it. Aka you cannot just go like "get all settlements, connect them all", cause you have nowere near enough settlers to use for traderoutes AND managing crops.