Well I'm no expert on this, but a quick search on the internet (at least looking at more recent posts) some explicitly state that the only way to do this is through the API, as Anton also says it works.
With WhatsApp there's a special "jid" element you can specify with the phone number to directly pass your message/files through intent. I got that from someone on the AM-WA group who had searched through the WA-manifest and found it. Haps there's something similar, "unofficial" in Telegram, but I doubt it. It might just be a privacy thing, or since they've got their API why bother with...
Well, apart from using the API, I only see two "manual" automisation solutions as possibilities:
a) with the intent as posted here and then use the Interact blocks to hit the right chat... I've seen flows on the community that do that for WhatsApp, so it might also work for Telegram.
b) use the other intent with the telegram link and id as Hendrik posted (with link), that will open the chat, and then use the Automate keyboard to type in the message (you'd probalby still have to use the interact block to hit the message field, and after typing in the message, again to hit the send button).
If anyone has some better info/solution?