RewriteEngine On
RewriteBase / 
##### ABOVE THIS POINT IS ALREADY INSERTED BY WORD PRESS
##### Michi’s code is BELOW #####
RewriteCond %{REQUEST_URI} wp-admin/
RewriteCond %{QUERY_STRING} !YOURSECRETWORDHERE
RewriteRule .*\.php [F,L]
RewriteCond %{QUERY_STRING} !YOURSECRETWORDHERE
RewriteRule ^ADMINFOLDER/(.*) wp-admin/$1?%{QUERY_STRING}&YOURSECRETWORDHERE [L]
##### Michi’s code is ABOVE #####
##### BELOW THIS POINT IS ALREADY INSERTED BY WORD PRESS
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]