1. Encode (Used for GET calls alone) the below values thereby replacing the Account Name as per yours :
{
"operation":{
"details":{
"accountname": "Sample Account",
"sitename": "0"
}
}
}
2. The site name value is set as "0", which will list the Requesters of all sites available in that particular Account. In future, you can change the value of the site name from "0" to your desired site name to list "Site Specific" Requesters.
3. Please make sure to use "data=" in front of the parameter as highlighted below: