How do you linking to an optional Masters without adding it to the .esp's Masters list

[Good0Provider]

How do you link against forms/factions/etc in optional Mods without the optional mod's esp file getting added to the Master's list?

 

Faction CoolModFaction Auto <-- how to link this in Creation Kit?

  

[PeterMartyr]

Game Get Form From File

https://ck.uesp.net/wiki/GetFormFromFile_-_Game

Use that code to fill the property or private variable, not the creation kit, Do not forget to cast it as a faction.

EDIT

Now that it is a soft master, if that code run, and the mod it is referencing is not in the load loader, it will create a papyrus error in the log, you might want to check the mod is installed first too, and that requires SKSE. Else let it go, the error will get caught by game engine for just vanilla presay, without causing any problems, with the exception of pedantic users that run the papyrus log for no reason whatsoever. They will get shitty 

[Good0Provider]

Considered using this method, but this will fail if the mod changes enough that the faction's id changes.

But if this is the state of the art method, so be it.  Thanks again. 

String CoolModFile = "CoolMod.esm"
if Game.GetModByName(CoolModFile) != 255
    CoolModFaction = Game.GetFormFromFile(0x000DE45, CoolModFile) as Faction
endif

 

Edited February 10 by Good0Provider

[scorrp10]

If you set your mod to use another as a master,  and have your mod reference something from it, it still does it by FormID.    That is, it will go for FormID 0xDE45  to fetch that Faction.   If that other mod gets updated so significantly that 0xDE45 is no longer that Faction, the master method will no longer work as well.    You would need to:

Download and install the updated master
Load your mod in CK with the updated master.   And hope you don't get a crash. 
Go over all the cases where you use any forms from the master, and correct.
Save your plugin and republish your mod.