Hey all 
I posted a reply to this thread a while ago after creating URL rewrites for whmcs, but I think that this deserves its own thread for people looking for it.
This is required as NGINX does not support the .httaccess.
Nevertheless here is the tutorial
https://hostup.org/blog/whmcs-friendly-urls-configuration-for-nginx/
If you have any questions, or if I have missed something in this simple, yet non existant elsewhere let me know on here or on the article itself
Also this is tested for latest WHMCS 7.7