how to rewrite a single URL to add .html in wordpress

I ported an old static HTML site to wordpress. All posts and pages now have permalinks /%postname%/

I need a single post URL to include the .html extension (it needs to match the original canonical URL which included the .html)

EXAMPLE:

www.domain.com/thispageonly/

-TO-

www.domain.com/thispageonly.html

This is needed for a single post only nothing more.

Thank you!