Parse the json payload from an IQA

I'm trying to parse through the payload returned from an IQA within c#. Is there a simple way to do this?
For example, I need the values of Joe and Smith from the below payload such that I can populate a textbox on a web form. Json.NET is our installed framework.
Thank you in advance for any help.

{
"$type": "System.Dynamic.ExpandoObject, System.Core",
"Items": {
"$type": "System.Collections.Generic.List`1[[System.Dynamic.ExpandoObject, System.Core]], mscorlib",
"$values": [
{
"$type": "System.Dynamic.ExpandoObject, System.Core",
"ResultRow": "2",
"UserName": "1234567",
"FirstName": "Joe",
"LastName": "Smith",
"EMAIL": "[email protected]",
"RegistrationTimeStamp": "2021-03-12T09:16:00",
"RegInterval": 49332
}
]
},
"Offset": 1,
"Limit": 1,
"Count": 1,
"TotalCount": 6061,
"NextPageLink": null,
"HasNext": true,
"NextOffset": 2
}