Go to the top of the page & click on "Private Messages" in the dark highlighted border ... that takes you to your profile page's PM section.
In the top left menu box click "Send New Message" ... that takes you to your outgoing message text area.
Start typing the member's screen ID name in the "Recipient Users" line ... just using the first few letters of the member's screen ID name ... and a menu of similarly spelled names will appear. Click on the appropriate member's name and that will appear in the Recipient Users line.
From there, just treat it like a Email or posted message ... then click on the "Submit Message" box at the bottom right of the text message area, when you're ready to send the PM.
There are some "options" that can be checked in the area below, and are self explanatory.
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Doing it this way insures that you get the right spelling of the member's screen ID name, as there are some members that have very similar sounding/spelled screen ID names. It also means that you can "pick" the member you want to PM, without having to search for a post of theirs to find their name ... especially if they don't happen to post a reply in your thread.
If you do click on a member's screen ID name ... a dropdown menu will appear, and you can click on "Private Message" in that menu and it will take you to your profile PM page and automatically add their name to the Recipient Users line.
... cp