"Loot source" is an enum. You can only select from the Loot Sources listed on the main page, like Barrel, BossMonster etc.
No idea what the dungeon plugin uses in terms of NPC. If it doesn't use NpcSpawn then the plugin wont support adding loot to the boss (unless you wanted to add the same loot to all scientist npcs of the same type).
If it is NpcSpawn, then you can use BetterNPC as the Loot source, and the name of the NpcSpawn profile for the prefab shortname, boss profile, event name field.
I assume Loot source having the paths to the NPC prefabs would be throwing an error when the plugin loads.