SMS Gateway for Android produces HTTP 500 Error

I'm trying to use the Traccar SMS Gateway for Android (https://github.com/traccar/traccar-sms-gateway) and interact with it using powershell. However, I seem to get a 500 response when I try to send an SMS. Any suggestion or where I can look up what might be causing the 500 code?

If I mess up the token the error changes to 401, so at least the authentication part must be working (it's my assumption). Here's the code:

$params = @{"to"="+4915229591111";"message"="Test";}
$header = @{"Authorization" = "ede92604"}
Invoke-WebRequest -Uri "http://192.168.26.141:8082/" -Method POST -Body $params -Headers $header

Unfortunately I get:

Invoke-WebRequest -Uri "http://192.168.26.141:8082/" -Method POST -Body $params -Headers $header
Invoke-WebRequest : The remote server returned an error: (500) Internal Server Error.
At line:5 char:1
+ Invoke-WebRequest -Uri "http://192.168.26.141:8082/" -Method POST -Bo ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-WebRequest], WebException
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand

Ideas and suggestions welcome. :)

It doesn't seem like you're sending in the right format. You need to send JSON data.

It doesn't seem like you're sending in the right format. You need to send JSON data.

Oh, thanks for the hint and you're absolutely right. With a very small change to the code it starts working. Pretty nice. This is the "working" code if anyone's interested:

$params = @{"to"="+4915229591111";"message"="Test";}
$header = @{"Authorization" = "ede92604"}
Invoke-WebRequest -Uri "http://192.168.26.141:8082/" -Method POST -Body ($params|ConvertTo-Json) -Headers $header