Converting Curl API Call to Invoke-WebRequest in PowerShell 5 - powershell

Question

I have done a bunch of searches, but nothing so far as helped me solve this. Any help is appreciated.
I am trying to convert a curl command to Invoke-WebRequest, but for some reason I am not getting a response back. I have tried:
$headers = #{
outh_a = "WEBREQ";
oauth_key = "key";
etc...
}
Invoke-WebRequest -Uri $url -Method POST -Body "{}" -Headers $headers -Verbose
The curl command looks like this:
.\curl.exe -x POST -d "{}" -H "oauth_a: WEBREQ" -H "oauth_key:key" -H "Content-Type: application/json" -H "oauth_sm:HMAC-SHA2" -H "oauth_t:timestamp"-H "oauth_s:key+signature=" "https://example.com/services/api/request?t=data&a=data2&n=404&i=0"
The command works perfectly and returns the data. I am using PowerShell as we want to parse through the data once received. Any help is greatly appreciated.
The curl command will fail if I submit it without -d "{}", so that part is required. The server I guess is expecting to receive a specific amount of data.
I am not sure what is going on to prevent a response. I have tried curl from the same machine I am doing the PowerShell script on and it works. I even used SoapUI to make the same call and it works there too.
Edit:
[System.Net.ServicePointManager]::CertificatePolicy = New-Object TrustAllCertsPolicy
[System.Net.WebRequest]::DefaultWebProxy.Credentials =
[System.Net.CredentialCache]::DefaultCredentials
[Net.ServicePointManager]::SecurityProtocol =
[Net.SecurityProtocolType]::Ssl3, [Net.SecurityProtocolType]::Tls,
[Net.SecurityProtocolType]::Tls11, [Net.SecurityProtocolType]::Tls12
$Domain = "https://example.com/services/api/request?t=oauth_auth_token&a=appid&n=404&i=0"
# OAUTH Authorization Request Header
$Header = #{
o_nonce = 'TEST'
o_con_key = 'key'
o_sig_meth = 'type'
o_timestamp = 'timestamp'
o_sig = 'signature'
o_ver = '1.0'
o_realm = 'http://example.com/services'
}
$Body = #{}
$API_Response = Invoke-RestMethod -Method POST -Headers $Header -Uri $Domain -Body $Body -Verbose -TimeoutSec 20

Answer 1

Update:
#MikeW I'd have to look at your PS code in more detail, could you post the entire API call minus the sensitive information (api key etc...)?
Also, on a side note, sometimes just passing the parameters on the URI works just as well. Have you tried passing the parameters through the URI? You can see the following example of an API call I'm making using just the URI:
Invoke-RestMethod -Uri ("$EndpointURL/Tickets/$TicketID/?username=$ServiceAccount&apiKey=$APIKey");