If this solution is from below post, it doesn’t work for html templates.
One way is to reply via Vbscript (using Start Process or PowerShell)
Const olFolderInboxItems = 3
Dim Items
Dim objSubject
Dim Arg
Set Arg = WScript.Arguments
Set objOutlook = CreateObject("Outlook.Application")
Set objNamespace = objOutlook.GetNamespace("MAPI")
Set objInbox = objNamespace.GetDefaultFolder(olFolderInboxItems )'Inbox
Set Items = objInbox.Items
Items.Sort "[Received]",True
ReplyEmail(Items)
Sub ReplyEmail(objItems)
For Each objItem in objItems
objSubject = Arg(0) '1st Argument as Subject from Start Process Activity
objsubject = objItem.subject
if Instr(objsubject,objSubject) <> 0 then
Set olReply = objItem.ReplyAll
olReply.HTMLBody = "Hello, Reply from Robot. Thank you. " & vbCrLf & olReply.HTMLBody 'Append Body
olReply.Send
Exit For
end if
Next
End Sub