DM Job calling through API

Hi - I am trying to call the DM job using RESTAPI call from Powershell using Basic Authentication . It is an On-Premise Environment and Version is 6.6. I know I can use the task scheduler but I requirement is to use the API. 

I have a PS Script but it failing and also failing in Postman with 404. The below is the script

$User = ""
$Token = ""
$base64AuthInfo = [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes("$($User):$($Token)"))
$Header = @{
Authorization = "Basic $base64AuthInfo"
ACCEPT="application/json"
}

$ContentType = "application/json"
$Url = "http://Host:Port/api/DataManagement/ExecuteStep"
$Body = @{
SequenceName="Sample"
BaseWebServerUrl="http://Host:Port/OneStream"
ApplicationName="Sample"
}

$json = $body | Convertto-JSON
Invoke-RestMethod -Method 'Post' -Uri $url -Headers $Header -Body $json

Any help would be appreciated.

Thanks

Krishna