Found solution, it's not pretty, but it works:
$wshell = New-Object -com WScript.Shell
$wshell.Run("iexplore.exe $url")
Start-Sleep 1
$wshell.sendkeys("$username")
$wshell.sendkeys("{TAB}")
$wshell.sendkeys("$password")
$wshell.sendkeys("{TAB}")
$wshell.sendkeys("{ENTER}")
if I find a better solution, I'll make sure to post it, or feel free to post your own :P