You can add Password in your Post by changing the Post Visibility to Password Protected from Public.
Navigate to Publish at right side > Visibility: Public EDIT > Password protected > type the desire password.

By changing the Visibility, the Post would require a password to access into it. Without password, no one can see the post.

This tool is awesome for the users who wants to protect their every post and want to show it to the specific people who do know Post Password.

Whenever reader try to read the post, he would need password. And following message will appear with a password field.
“This post is password protected. To view it please enter your password below”