How can i PM?

[tortugarabioso]

How can i reply/send messages?

[Idly]

Type /tell [player name] and your message. For anymore commands type /help

[obee]

/r for replies

[Candylicks]

You need to make 10 forum posts before you can pm on forums.

[Schnitzel]

You need to make 10 forum posts before you can pm on forums.

Adding to that,
After you reach 10posts

On the sts forum website (not mobile app),
On the top right corner, click Settings
Scroll down and on the left should be something called "general settings" click that
Then in the middle of the page, scroll down until you see "private messaging"
Click enable

You should now be able to private message others on forums