Take a link of that particular TG post, share the post link here. If moderator wants, they can verify that whether the post was plagiarized.
This would only be possible in groups which are public, but would not apply to secret groups or private messages. Also messages on telegram can be edited/deleted and it isn't yet fully indexable, so proving the original source is going to be difficult.
If the prove cannot be verified then it should not be accepted, and if effected it would open a can of worms with lots of hugely debated appeal threads.