Hello Nishit we have got a reply from seniour developer on your error mentioned erlier:
As team say your request should be as below:

{
“privateKey”: “hidden”,
“portInOrder”: {
“desiredDueDate”: “2023-08-29”,
“tnList”: {
“tnItem”: [
{
“tn” : 9838888880,
“accountNum”: “3777”,
“atn”: “0123456789”,
“authName”: “Test”,
“authDate”: “2023-08-28”,
“accountPin”: “9009”,
“endUser”: {
“name”: “Nano Telecom”,
“streetNum”: “20900”,
“streetName”:”30th”,
“city”: “Miami”,
“state”: “FL”,
“postalCode”: “33180”,
“typeOfService”: “B”
}
}
]
}
}
}

If you noted the first line has a capital “K” but in your code it has got small “k” in “privateKey” word.
{
“privateKey”: “hidden”,

I can not find any othre diffrence from the code you provided and the code siniour replys with so hoping it will work now can you please let us know if any other issues.

Let us know if any updates on Custom Features.
Cheers