$homePage = Invoke-WebRequest <ftp https url>
$username = "<username here>"
$password = "<password here>"

$form = $homePage.Forms[0]
$form.Fields["username"] = $username
$form.Fields["password"] = $password

$portalPage = Invoke-WebRequest -Uri $Form.Action -Method POST -Body $Form.Fields -SessionVariable ftp
Invoke-WebRequest -Uri "<link to Excel Sheet I want to downlaod" -OutFile <output directory>