Powershell Invoke - RestMethod - powershell

Question

I would like to use an POST request with powershell but am getting the following errormessage. I have searched many hours, but can't find the problem. The json body seems also to be correct(?).
Thanks in advance for every help!
$AllProtocols = [System.Net.SecurityProtocolType]'Ssl3,Tls,Tls11,Tls12'
[System.Net.ServicePointManager]::SecurityProtocol = $AllProtocols
(New-Object System.Net.WebClient).Proxy.Credentials = [System.Net.CredentialCache]::DefaultNetworkCredentials
$body = #{
"client"="myKey"
"request-type"="authentication"
"username"="myUsr"
"password"="myPwd"
} | ConvertTo-Json -Depth 10
$header = #{
"Content-Type"="application/x-www-form-urlencoded"
}
$res = Invoke-RestMethod -Uri "https://myUrl:4000/abc" -Method 'POST' -Body $body -Headers $header
Invoke-RestMethod :
Error
Cannot POST /abc
In Zeile:1 Zeichen:8
+ $res = Invoke-RestMethod -Uri "https://myUrl:400 ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebExc
eption
+ FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand
Edit:
I have edited my code but get the same error:
$AllProtocols = [System.Net.SecurityProtocolType]'Ssl3,Tls,Tls11,Tls12'
[System.Net.ServicePointManager]::SecurityProtocol = $AllProtocols
(New-Object System.Net.WebClient).Proxy.Credentials = [System.Net.CredentialCache]::DefaultNetworkCredentials
$body = #{
"client"="mykey"
"request-type"="authentication"
"username"="myusr"
"password"="mypw"
}
$res = Invoke-RestMethod -Uri "https://myUrl:4000/abc" -Method 'POST' -Body $body
And the 400 error is'nt a error code, I think it's the port.
Invoke-RestMethod :
Error
Cannot POST /abc
In Zeile:1 Zeichen:8
+ $res = Invoke-RestMethod -Uri "https://myUrl:400 ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebExc
eption
+ FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand
The request should look like the following:
client=sadsda42424242&request-type=authentication&username=extern&password=geheim
Also I have to use the content type „application/x-www-form-urlencoded“.