So I'm pretty sure you don't need to post a certain amount of times to send a PM. The PM just doesn't show up in your sent items unless you change your settings to make it show up.
To do so go to User CP in the top left
Edit Options (Under Settings & Options)
Check "Save a copy of sent messages in my Sent Items folder by default
I only know this cause I was so confused as to why I was sending PMs and they didn't show up in my sent items