While foxes cannot be tamed, they can come to "trust" a player. In vanilla gameplay this happens when a player breeds a pair of foxes with sweet berries - their baby will trust the player.

NBT

TrustedUUIDs: The players that the fox trusts. For a list with more than 2 elements, only the first and the last are considered.

• The UUID of each trusted player.
  •  L: Least significant bits of the Universally Unique Identifier of the friend, to be joined with M to form the friend's unique ID.
  •  M: Most significant bits of the Universally Unique Identifier of the friend, to be joined with L to form the friend's unique ID.

https://minecraft.gamepedia.com/index.php?title=Fox&section=14#Entity_data