CZNeo

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
RewriteCond %{HTTP_HOST} ^zelenenoviny\.cz [NC]
RewriteRule ^(.*)$ https://www.zelenenoviny.cz/$1 [L,R=301]

Thank you for the answer!

however, it seems as if nothing’s changed. When I put https://www.mydomain.cz in the address bar I am redirected to mydomain.cz

My goal was to do that the other way round. Anyway, I tried to tweak your code by switching Condition and Rule

RewriteCond %{HTTP_HOST} ^example\.com [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [L,R=301]

Didn’t work either.

Thanks for all the suggestions. In the end, my webhost just said that he allowed wildcard but he did not ??