I try to create a collection in CosmosDB and I don't know how to create a good shardkey !

I had something like that in mind, but it does not accept my shard-key !

{
"shard_key" : ["50.836421", "4.355267"],
"position":
  {
    "type": "Point",
    "coordinates": [50.836421, 4.355267]
  },
}

Does someone has experience with this?