There are many ways to do it, but if you want to do it in a way that makes sense in-game you need to place something in Fort Joy Itself that you can use to travel there. A portal, a door, cave entrance, something that like that.
So if you haven't already, add Origins as a dependency in the project settings. After doing so, I would save the mod and completely reload the editor just to be safe.
Also you need to place a point trigger in your level, give it a logical name like Point_Teleport_MyLevel, and make it global on the side bar.
Open the story editor and run "generate definitions" from the menu.
Then on the object in fort joy, go to the sidebar and select an "OnUseAction". Select teleport. That will give you a list of options you can pick, select trigger and find your trigger in your level in the list. Pick it and you should be good to go.
Then you'll still need to the the same steps above, only in reverse so you can get back to Fort Joy.