The item ID for Legend Fish in Stardew Valley is:
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  and press OK. You will then receive the legend fish 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 legend fish item.
If you need more help with spawning legend fish, we have a detailed guide on our blog - click here to visit that page.
The Legend Fish is one of 5 legendary fish (Crimsonfish, Angler, Glacierfish, Mutant Carp), the only one of which that can be caught multiple times. Catching a Legend Fish requires level 10 fishing which is not surprising considering its sell price is the highest in the game (for a single item).
The king of all fish! They said he'd never be caught.