.htaccess
##Add a Trailing slash
# FROM: http://was-mit-internet.de/unterseite/filename.html
# TO: http://was-mit-internet.de/unterseite
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ https://www.domainname.tld/$1/ [L,R=301]