Suppose you are writing a blog post and now you want your users to send a message to you about that blog post. You can add this link in that blog post. Now when a user will click that link they will redirect to message page with your name in the “To” field so that they will not need to search your name. They can just write subject and message and send to you.

I believe you are using version 5.1+.There are 2 shortcodes to easily achieve this. I suggest you use one of those instead of using link directly. You can get link of those shortcodes in little lower of that docs page https://www.shamimsplugins.com/docs/front-end-pm/customization/user-name-pre-populate-new-message/.