The item ID for Monster Musk in Stardew Valley is:
879
The above cheat code applies to all platforms including PC/Mac (Steam), XBOX One, PS4 and the Nintendo Switch.
Spawning items in Stardew Valley is not done via commands, but rather through a cheat in which you must set your character's name, or the name of an animal, to the item code of the item you wish to spawn surrounded by [ and ].
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 [879] and press OK. You will then receive the monster musk 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 [879] 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 monster musk item.
If you need more help with spawning monster musk, we have a detailed guide on our blog - click here to visit that page.