TicketCreate.ps1

$user = "John.Doe"
$pass = "VerySecret"
$uri  = "https://FQDN/otrs/nph-genericinterface.pl/Webservice/GenericTicketConnectorREST"

$headers = @{}
$headers.Add("Accept", "application/json")
$headers.Add("Content-Type", "application/json")

# Create Ticket
# Parameter see https://github.com/znuny/Znuny/blob/dev/Kernel/GenericInterface/Operation/Ticket/TicketCreate.pm#L69

# Read attachment and encode (please prepare one with the used filename)
$Content = Get-Content lorem-ipsum.txt
$ContentBytes = [System.Text.Encoding]::UTF8.GetBytes($Content)
$ContentEncoded = [System.Convert]::ToBase64String($ContentBytes)

$TicketData = @{
  UserLogin = $user
  Password  = $pass

  Ticket = @{
    Title = 'Tickettitle'
    QueueID = 1
    State = 'new'
    Priority = '3 normal'
    CustomerUser = 'do-not-reply@znuny.com'
    Type = 'Unclassified'
  }
  Article = @{
    Subject = 'The article subject'
    Body    = 'Test'
    ContentType = 'text/plain; charset=utf8'
    MimeType = 'text/plain'
    Charset = 'utf8'
  }
  Attachment = @{
      Content = $ContentEncoded
      ContentType = 'text/plain'
      Filename = 'lorem-ipsum.txt'
  }
}
$json = $TicketData | ConvertTo-Json
$Result = Invoke-RestMethod -Method Post -Headers $Headers -ContentType 'application/json' -Uri "$uri/Ticket" -Body $json
Write-Host Created ticket $Result.TicketNumber