Sometimes, you may need to have private message button, where users can contact with author of post. This is useful if you have multi author site and use BuddyPress (so, you need to have BP activated + enabled Private Messaging in Settings – BuddyPress. Next, add shortcode of button. You can find it in shortcode generator of theme. Instead of link of button, add name “buddypress”. Example
[wpsm_button color="green" size="medium" link="buddypress" icon="none" target="_blank" rel="nofollow"]Contact vendor[/wpsm_button]
You can also add Contact form to post if you need. Read article
How to add contact form in Post sidebar
By default, if you set shortcode in sidebar, it will show on whole site. But what if you want to show it only on single pages? No problem, use widget Rehub: Show only in post. Everything that you add to this widget will be visible only on single pages.