The slang term for posting deliberately provocative messages online is trolling.
Trolling involves sending inflammatory, insincere, off-topic, or otherwise provocative material to trigger emotional reactions, derail discussion, or upset a community. A person who does this is called a troll. The behavior can range from playful mischief to sustained harassment, and the key feature is usually the intention to provoke rather than simply expressing an unpopular opinion.
The term became established in early online communities and newsgroups during the 1990s. It is often confused with flaming: flaming is generally an angry online exchange, while trolling describes behavior intended to lure others into such a reaction. “Rage-baiting” is a related modern term, but it emphasizes content deliberately designed to generate outrage, often for attention or engagement.