$browser = New-Object System.Net.WebClient
$browser.Proxy.Credentials =[System.Net.CredentialCache]::DefaultNetworkCredentials
[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
$url = "https://login.salesforce.com/services/oauth2/token"


$postParams = @{
                grant_type = "password"
                client_id="the_id";
                client_secret="the_secret";
                username="my_username";
                password="my_password";
               }

$preAuth = Invoke-RestMethod -Uri $url -Method POST -Body $postParams -ContentType "application/x-www-form-urlencoded"
$preAuthToken = @{access_token = $preAuth.access_token}
$authValue = "Bearer $($preAuthToken.access_token)"
$headers = @{
    "Authorization" = "Bearer $($preAuthToken.access_token)"
}

$url2 = "https://na54.salesforce.com/services/data/v20.0/sobjects/"
Invoke-WebRequest -Uri $url2 -Method GET -Headers $header