I've already tried multiple time since I am the ambassador to Europe. Here is the email from the The Poland-Lithuania Commonwealth:
Only nations with Communication authority are permitted to post on Europe's RMB. This is mainly so that banned nations are unable to post RMB messages.
Edit: How would one go about this because communications description it says
"Can send region-wide telegrams without stamps, compose Welcome telegrams, suppress & unsuppress posts on the Regional Message Board, and control who can recruit for the region."
It doesn't say can allow people to post in RMB
The option to control RMB posting is under embassies, I believe. I don't have control of that, so I wouldn't know for sure.
I would recommend making a puppet (maybe "Ladavia in Europe" or something) and using that to communicate on the RMB. I have a dedicated puppet for each of my ambassadorships, and I find it a lot easier to manage than just using 1 nation across regions.
Also, as ambassador, you don't have to just be on the RMB. You can introduce yourself on Discord and any offshore forum, which Europe happens to have. Consequently, you should probably set up a thread on said forums here: http://www.europe-game.eu/index.php?forums/foreign-office.118/
Taken from the Admin page, "Nations in embassy regions are permitted to post on the Forest Regional Message Board."
So any nation in a region with an embassy with us can post, but regions we have embassies with don't have to do the same for us.
Europe has set their RMB so that only nations who have communications powers (read: are in the government) of their embassy regions can post on their RMB if they're not residents. You are a regional officer of Forest with communications powers, so you can post on Europe's RMB. Cosona and New Ladavia are not, so they can't.
It really comes down to the fact that if we allow all nations from embassy regions to post RMB messages, than banned nations (which go to TRR, whom we have a embassy with) will still be able to post messages on, and harass, our RMB.
I strongly suggest that all ambassadors from abroad send an ambassador puppet.