To create a custom account field, the following permissions are required.

Account permission: the user should have permission to manage account.
When updating a field value for a multivalued field (checkbox, multiselect), values must be strings in an array, e.g. ["Option 1", "Option 2", "Option 4"], whether one value or multiple are being set.

📘
Allowed datetime format is ISO only

{
  "accountCustomFieldDatum": {
		"fieldValue": "New title"
  }
}

{
    "accountCustomFieldDatum": {
        "id": "1",
        "accountId": 2,
        "customFieldId": 5,
        "createdTimestamp": "2018-10-29 20:40:38",
        "updatedTimestamp": "2018-11-06 02:21:36",
        "fieldValue": "New title",
        "links": {
            "account": "https://:account.api-us1.com/api/:version/accountCustomFieldData/1/account",
            "accountCustomFieldMetum": "https://:account.api-us1.com/api/:version/accountCustomFieldData/1/accountCustomFieldMetum"
        }
    }
}