@Vineet_Chauhan the payload wasn't using valid JSON. Please try using below payload in your POST:
{
"resource": [{
"id": 0,
"FirstName": "Vineet",
"LastName": "Chauhan",
"username": "sdsds",
"Email": "sdsds.sdsd @gmail.com"
}],
"ids": [
0
],
"filter": "string",
"params": [
"string"
]
}