Using this command you'll be able to get a monster spawner that is spawning Skeletons when you place it down.

/give @p minecraft:spawner{BlockEntityTag:{SpawnPotentials:[{Entity:

,Weight:1}],SpawnData:{id:"minecraft:skeleton"}}}

The problem here is:

 - The monster spawner, when you place it down in Creative mode, it works perfectally fine. However, when you placed said monster spawner in Survival Mode, the BlockEntityTag for that monster spawner isn't saved, and it reverts to a Pig Spawner (aka default spawner) instead. Please fix this issue as it is very vital to datapack creators who would wish to use this for a datapack that they're working on.