PowerShell v3 Invoke-WebRequest: Troubles with forms - forms

Question

Since I upgraded to Windows 8 a lot of my PowerShell scripts relying on launching an invisible IE won’t quite work anymore, so I tried switching to the Invoke-WebRequest command. I did a lot of googling but still can’t get my script to work.
This is what it should do:
load up a website with a simple form (username, password, submit-button),
enter the credentials
and submit them.
The Microsoft tech-net examples were not very helpful for me, that is what I pieced together:
$myUrl = "http://some.url"
$response = Invoke-WebRequest -Uri $myUrl -Method Default -SessionVariable $rb
$form = $response.Forms[0]
$form.Fields["user"] = "username"
$form.Fields["password"] = "password"
$response = Invoke-WebRequest -Uri $form.Action -WebSession $rb -Method POST
$response.StatusDescriptionOK
I receive two errors, the first one when trying to write into the user field:
Cannot index into a null array.
$form.Fields["user"] = "username"
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : NullArray
The second one has to do with the $form.Action which I have no idea what it should read:
Invoke-WebRequest : Cannot validate argument on parameter 'Uri'. The argument is null or empty. Supply an argument that is not null or empty and then try the command again.
Again, I relied heavily on example #2 at Microsoft.

Answer 1

Try doing the post directly e.g.:
$formFields = #{username='john doe';password='123'}
Invoke-WebRequest -Uri $myUrl -Method Post -Body $formFields -ContentType "application/x-www-form-urlencoded"

Answer 2

To address your problem with the unsigned/untrusted certificate, add the line
[System.Net.ServicePointManager]::ServerCertificateValidationCallback = {$true}
before the Invoke-WebRequest statement

Answer 3

The example in the question works, but you have to use rb and not $rb in the first line:
$response = Invoke-WebRequest -Uri $myUrl -Method Default -SessionVariable rb
I also had to use ($myUrl + '/login') since this is my login address.
$response = Invoke-WebRequest -Uri ($myUrl + '/login') -Method Default -SessionVariable rb
And in the last line used ($myUrl + $form.Action):
$response = Invoke-WebRequest -Uri ($myUrl + $form.Action) -WebSession $rb -Method POST

Answer 4

If you're me and have been troubleshooting a bad Web Request, in my case a -Body that was becoming null at my API, then you will want to know about the gotcha that is about interleaving your line continuations with comments. This
$r = iwr -uri $url `
-method 'POST' `
-headers $headers `
# -contenttype 'application/x-www-form-urlencoded' ` # default
-Body $body
Notice the commented out line # -contenttype 'application/x-www-form-urlencoded' # default
Putting a comment truncates the remaining back-ticked line continuation. Therefore, in my case my web request ended up with a request having 0-byte payload.