Read Comments Field with API

I do send an email with powershell through custom trigger.
I would like to send the comments which the customer adds to the request when he closes the request.
I read some fields like this:

$jsondata = Get-Content $json -Encoding UTF8
$obj = ConvertFrom-Json $jsondata

$subject = $obj.request.SUBJECT
$priority = $obj.request.PRIORITY
$ticket = $obj.request.WORKORDERID
$technician = $obj.request.TECHNICIAN
$wunschtermin = $obj.request.Wunschtermin
$auftraggeber = $obj.request.REQUESTER
$comments = $obj.request.XXYYZZ

in the last line I don't know the name of the comments-object
Could you tell me the name, please.
Do you have a list of all names, which I can read in my script?

Regards, Markus