I realize this is an old post, but this is the first Google result for "unRAID Telegram", and I found @Bokonon's post above remarkably helpful, and I'd like to contribute a bit more. As noted above, upon restart, the changes made to NotificationAgents.xml are erased. To get around that, we can use a brittle, but functioning shell script:
# Rename current NotificationAgents.xml.
mv /usr/local/emhttp/webGui/include/NotificationAgents.xml /usr/local/emhttp/webGui/include/NotificationAgents.old
# Insert Telegram chunk and redirect out to new NotificationAgents.xml file.