A note on the endpoint:
bulkCreate updates Last modified date of associated account. It is common case that collection items in bulkCreate request contains same accountId(s). So, the first item’s accountId only will be used for updating Last modified date of the account. Please note that If there are multiple different accounts provided in same bulkCreate request, not all of the account’s modified date will be updated.

[
   {
       "accountId": "1",
       "customFieldId": "1",
       "fieldValue": "test title"
   },
   {
       "accountId": "1",
       "customFieldId": "2",
       "fieldValue": "test title"
   },
   {
       "accountId": "1",
       "customFieldId": "3",
       "fieldValue": [
          "option 1",
          "option 3",
          "option 4"
      ]
   },
   {
       "accountId": "1",
       "customFieldId": "4",
       "fieldValue": 9999,
       "fieldCurrency": "cad"
   },
   {
       "accountId": "1",
       "customFieldId": "5",
       "fieldValue": "100.99"
   },
   {
       "accountId": "1",
       "customFieldId": "6",
       "fieldValue": "2018-12-31"
   },
  {
       "accountId": "1",
       "customFieldId": "7",
       "fieldValue": "2020-05-19T02:45:00-05:00" //ISO format
   }
]

{
    "message": "the bulk insert was successful"
}