curl -i -X POST "https://api.box.com/2.0/storage_policy_assignments" \
     -H "authorization: Bearer <ACCESS_TOKEN>" \
     -H "content-type: application/json" \
     -d '{
       "storage_policy": {
         "type": "storage_policy",
         "id": "1434325"
       },
       "assigned_to": {
         "type": "user",
         "id": "9987987"
       }
     }'

{
  "id": "ZW50ZXJwcmlzZV8xMjM0NTY3ODkw",
  "type": "storage_policy_assignment",
  "storage_policy": {
    "id": "11446498",
    "type": "storage_policy"
  },
  "assigned_to": {
    "id": "11446498",
    "type": "file"
  }
}