How to add a Object in JSON Array.

"itms":[
    {
       "num":1201,
       "itm_det":{
          "txval":84273.8,
          "rt":12,
          "camt":5056.43,
          "samt":5056.43,
          "csamt":0
       }
    }
 ]

List<Itm> CustomerInvoitem = new List<Itm>();
DataTable dt = GetData(string.Format("SELECT BILL_DATE,(SUBSTR(CUST_GSTIN,0,2))POS,(CGST_RATE+SGST_RATE+IGST_RATE)RATE,CGST,SGST,IGST,BILL_FREIGHT,NET_AMNT FROM GST_SUMMARY_VW WHERE  BRANCH_GSTIN='{0}' AND CUST_GSTIN='{1}' AND BILL_NO='{2}'    ORDER BY BILL_NO ", BranchGst, CustomerGst, InvNo));
for (int i = 0; i < dt.Rows.Count; i++)
{
    CustomerInvoitem.Add(new Itm
    {
        num = 1201,
        itm_det = {Convert.ToDouble(dt.Rows[i]["BILL_FREIGHT"]) }
    });
 
}
return CustomerInvoitem;