Add Reply to Discussions List by Powershell
Add-PSSnapin microsoft.sharepoint.powershell -ea 0
$webUrl = "https://yourCommunitysiteUrl"
$viewUrl = "/Lists/Community%20Discussion"
$parentId = 1;
$web = Get-SPWeb $webUrl
$listUrl = $web.ServerRelativeUrl + $viewUrl
$list = $web.GetList($listUrl);
$list.Title
$parent = $list.GetItemById($parentId);
[System.Reflection.Assembly]::LoadWithPartialName(“Microsoft.SharePoint")
$item = [Microsoft.SharePoint.Utilities.SPUtility]::CreateNewDiscussionReply($parent)
$item["Body"] = "My PS body";
$item.Update();