So you want to create your own point of interest that has a chance to randomly spawn?
First of all you have to find a place for your new point of interest, open up WorldData > Blocks on Creation Kit, double click a block and click preview button in the inspector to see how it looks like:
Find a spot you like and press Control + H to open terrain cutting tools, pick an area, in my case I'm making it 3x3, give it an ID as well. Click create overlay when you are satisfied.
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Find your new overlay area in WorldObjects > WorldSpace, right click it and view in inspector then add the BSGPlanetContentManagerContentProperties_Component component to it.
Go under WorldData > Location, right click and create a new location for your POI, add the keywords LocTypeOE_Keyword and LocTypeOverlay:
Go back under WorldObjects > WorldSpace, double click your overlay and add the newly created location to it:
I recommend adding a MapMarker that is already discovered to your location so you can find it quicker in game:
Now go under WorldData > Planet Content Manager Tree, open PCM_BlockCreationRequest and find PCM_BlockCreation_DRsRLs, right click and add worldspacecontent, find yours on the list and add it:
Here I have removed every other one to make sure it spawns easily, make sure to save your mod and back it up before you remove anything!
That's it, save the plugin, start the game, enable the plugin and go see if it spawns in! You can also manually place it on the planet with the console command AddWorldSpaceToPlanetBiome WorldSpaceName PlanetName biomeIndex
But that doesn't guarantee it actually spawns randomly in.
Edit:
To add random encounters into your overlay you can drag in the PackIn called RE_PackIn_Overlay and arrange the triggers and markers around your POI. After this you sometimes get random encounters when traveling to it. You can remove some triggers to disable some encounters, for example if you do not want spaceship encounters remove all the landing areas.