Finally, I found the solution :
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{QUERY_STRING} .
RewriteRule ^newsletter-(.+)$ https://www.datingwatch.org/newsletter-$1? [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
Hope this could help