test.ps1

[string]$doc = @"
{
    "parameters" : {
        "valuename": {
            "value": "123"
        }
    }
}
"@ 

#$json = $doc | ConvertFrom-Json

[Reflection.Assembly]::LoadFile("C:\Development\Newtonsoft.Json.dll") | Out-Null
$json = [Newtonsoft.Json.Linq.JToken]::Parse($doc)

$json.parameters | % {
    [string]$value = $_.value.value
}