thanks for using Rebex products!
Here is a powershell script that connects to the Ftp server on port 990 with implicit TLS/SSL mode:
Add-Type -Path "c:\Program Files (x86)\Rebex Components 2015 R3.1\bin\net-4.0\Rebex.Ftp.dll"
$ftp = New-Object Rebex.Net.Ftp
FYI, Rebex.Net.SslMode is an enum, working with enums from PowerShell is a bit tricky and is discussed e.g. at this Microsoft site.