Look at how the files: CompostBin, PrimalINventoryBP_CompostBin, and PrimalItemConsumable_Fertilizer_Compost works, the keyword in those files is Auto Craft.
In the PrimalItemConsumable_Fertilizer_Compost I can see where “Crafting Requires Inventory Component” links it back to the compost bin, but oddly enough the metal ingot doesn’t seem to have that link with the forge.