How can I add “.html” to the end of a single URL on a WordPress website?

You can do this quickly by creating a rule in .htaccess.

Log in to WordPress and go to the page you want to link to.

Then click “preview changes”.

Look for the preview_id parameter in the url.

And use this id after de index.php?p= in the .htaccess like:

# BEGIN WordPress

RewriteEngine On
RewriteBase /

RewriteRule ^mypage/page.html$ index.php?p=262 [L]

RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

# END WordPress