@Rahul_S Instead of passing values in parameter, you can pass the entire JSON as string in Body property of “HTTP request” activity.( Please see attached screenshot)
Note: Make sure the json is converted in single line string. If needed escape double quotes with one more double quote.
In your case pass body as string:
{““webServiceReq””:{““channelId””:““RPA””,““cifId””:““123456789"”,”“serReqId”“:”“getlist”“,”“requestUUID”“:”“12345678"”,““request””:{““custId””:"“0122345678"”}}}