add2DoYo.pl builds this item JSON document...
$data = qq[
	{
		"holdingsRecordId":"5ada1ed5-d9c4-4fbe-8110-aa3b0b23bbee",
		"materialTypeId":"7b0b1c12-73d5-42f6-a8de-6d93d151bcc5",
		"permanentLoanTypeId":"84ff8075-36d1-4156-82f3-3485ce8affab",
		"status":{"name":"Available"}
	}
];
and POSTs it to /item-storage/items like so:
curl -s -X POST $headers -d '$data' $baseURL/item-storage/items
{
  "id" : "5547e0f2-29e5-4eba-ba12-f061ffdb4f61",
  "_version" : 1,
  "hrid" : "it00001161103",
  "holdingsRecordId" : "5ada1ed5-d9c4-4fbe-8110-aa3b0b23bbee",
  "formerIds" : [ ],
  "effectiveCallNumberComponents" : { },
  "yearCaption" : [ ],
  "administrativeNotes" : [ ],
  "notes" : [ ],
  "circulationNotes" : [ ],
  "status" : {
    "name" : "Available",
    "date" : "2023-10-09T18:34:15.244+00:00"
  },
  "materialTypeId" : "7b0b1c12-73d5-42f6-a8de-6d93d151bcc5",
  "permanentLoanTypeId" : "84ff8075-36d1-4156-82f3-3485ce8affab",
  "effectiveLocationId" : "96f6675a-9854-49a9-80cc-6e3e2caf1ece",
  "electronicAccess" : [ ],
  "statisticalCodeIds" : [ ],
  "metadata" : {
    "createdDate" : "2023-10-09T18:34:15.244+00:00",
    "createdByUserId" : "308a01e4-1108-4ea3-aa25-8b1b32cf7643",
    "updatedDate" : "2023-10-09T18:34:15.244+00:00",
    "updatedByUserId" : "308a01e4-1108-4ea3-aa25-8b1b32cf7643"
  }
}