This is not really an in-game topic, but relates to the current STO forums.
If you need to send a PM to another forum user, you can't simply click on their profile link next to their post. That will take you to Core Connect, which is currently unused by STO players because it's not fully implemented for STO yet.
To PM me or another forum user, just copy their username and go into the User CP link near the top of the forums.
That puts you into the forum control panel for your user account. Under private messages, click "Send New Message", put in the username of the person you want to contact and send your message. The Recipient Username(s) field has autocompletion and will show you the member names that match what you're entering.
There are additional options you can select for each message, including saving it to your sent items folder and showing your signature.
Just a reminder, PM's are no excuse for violating forum rules... forum rules apply there, too.
If you get a PM that violates forum rules, you can either ignore it and delete it, or report it. DON'T REPLY TO IT. Community Mods can't moderate PM's, so you'll have to contact PWE Customer Support or the Community Manager to report a problem.
There is a limit to how many messages can be stored in your message folders. There is one folder for received messages and one for sent messages. If you PM a lot, it's a good idea to click the "Empty Folder" link occasionally.
If you want to save some of those messages, you'll have to either select the messages you want to delete manually, or delete the messages one-by-one. By and large, it's probably easier and more efficient to just whack them.
Volunteer Community Moderator for the Star Trek Online forums -- My views may not represent those of Cryptic Studios or Perfect World Entertainment. If you wish to speak to someone on the community team, file a "forums and website" support ticket here, as we are not able to respond to PMs regarding moderation inquiries. Link: How to PM - Twitter @STOMod_Bluegeek