Not if it's disguised as a private individual (which is the default assumption in discussion forums such as this one).
If a company wants to present its views, they can do so via press releases or clearly mark their posts as such (or via verified accounts like on Twitter).
Honest question: under which law? Does anything actually prohibit companies from employing people from going on forums and shilling? I think it's perfectly fine for the third party platform to have a policy forbidding this, but the government enforcing something like this would not be ok.
I didn't answer the question with laws in mind, as the parent wasn't exactly asking a legal question, but what I would guess would be more a moral one.
IINAL, but since we are at the topic of laws, in the last years influencers (at least in Germany) had to fear hefty fines if they didn't label sponsored posts as such, as that counts for covert advertising. As astroturfing could be construed as advertising, that might fall under the same law.
If a company wants to present its views, they can do so via press releases or clearly mark their posts as such (or via verified accounts like on Twitter).