any php page? post can have author but every php page does not have author, So how will you get author if do not have one?
Do you want to send message to a constant author?
If i understand you correctly can you put what suggested here https://www.shamimsplugins.com/docs/front-end-pm/customization/user-name-pre-populate-new-message/
Change USER with get_the_author_meta( 'user_nicename')