curl -i -X POST "https://api.box.com/2.0/legal_hold_policy_assignments" \
     -H "authorization: Bearer <ACCESS_TOKEN>" \
     -H "content-type: application/json" \
     -d '{
       "policy_id": "123244",
       "assign_to": {
         "type": "folder",
         "id": "6564564"
       }
     }'

{
  "id": "11446498",
  "type": "legal_hold_policy_assignment",
  "legal_hold_policy": {
    "id": "11446498",
    "type": "legal_hold_policy"
  },
  "assigned_to": {
    "type": "folder",
    "id": "6564564"
  },
  "assigned_by": {
    "id": "11446498",
    "type": "user",
    "name": "Aaron Levie",
    "login": "ceo@example.com"
  },
  "assigned_at": "2012-12-12T10:53:43-08:00",
  "deleted_at": "2012-12-12T10:53:43-08:00"
}