rewrite wordpress login url when it contains an affiliate ID

Solution: 

You can try with this:

RewriteEngine On
RewriteRule ^([a-zA-Z0-9_-]+)/wp-admin$   /wp-admin [L,NC]

It redirect any given path followed by /wp-admin to yourdomain.com/wp-admin
ex.
www.yourdomain.com/anything/wp-admin
redirect to:

www.yourdomain.com/wp-admin