Hi!
Thanks for the question! 
Yes, you can create a thread here related to companies you can invest in. The opening post of the thread should be comprehensive and well-written, containing enough information, not just a short scribble of a few lines.
Good that you mentioned marketing. Sometimes there are usernames that indirectly market some services and post links to loan services or similar, so as you guessed, that’s not okay, but you weren’t planning to do that anyway.
If you have problems or questions, you can ask, for example, in these threads:
https://keskustelut.inderes.fi/t/kysy-vastaa-ketju-sijoittamisesta/
https://keskustelut.inderes.fi/t/keskustelua-valittajista/
And sometimes you can get help with something from the Coffee Room (Kahvihuone) here, and you don’t necessarily always need a separate thread for an issue that might not generate enough investment-related discussion. You can create a thread once you have browsed the Forum enough; initially, when you’ve just joined the Forum, you can’t create a thread, but the Forum’s platform automation automatically grants rights once you’ve been on the Forum long enough.
You can create a thread as long as it brings enough added value to investors + as long as there’s nothing vague in it and as long as the opening post of the thread is good enough + comprehensive. Sometimes a thread is not needed if the topic is small and doesn’t generate much discussion; often small topics/questions can fit under an already existing thread. In any case, before creating a thread, you should browse the Forum enough and read around to know what is expected here.
Thank you and happy reading!