where to add redirection rewriterule in .htaccess file?

Your external redirect needs to go at the very top of your .htaccess file, before the WordPress front-controller (ie. before the # BEGIN WordPress section).

If you place it at the end of the file then it will simply never get processed, because the WordPress front-controller routes all URLs and then stops further processing.

If you place it inside the # BEGIN WordPress block then WordPress is likely to (try to) overwrite this in a future update.

Aside: You don’t need the <IfModule mod_rewrite.c> wrapper. You already know that mod_rewrite is enabled on your server (or not) and presumably this redirect is mandatory?