The item ID for Goat Milk in Stardew Valley is:
To spawn this item using an animal's name, visit Marnie's Ranch, speak to Marnie and purchase an animal (we recommend a chicken as it is cheapest). Name the animal  and press OK. You will then receive the goat milk item.
You can only set your character's name once, at the start of the game (when you create your world), for this reason we recommend you use the animal name method. If you do wish to use this method, first set your character's name to  when creating your save. Whenever you speak to a villager that says your name in dialogue (for example Gus at the Stardrop Saloon) you will be given the goat milk item.
If you need more help with spawning goat milk, we have a detailed guide on our blog - click here to visit that page.
The milk of a goat.