that cant be achieve with a htaccess file. You need different things. possible solution could be: apache mod proxy so you could proxy everything through that url. the other way could be a php written some kind of proxy. Working with curl or and remote configured fopen.
But be careful to also send the post data and the cookies, otherwise the user session could not be handled if its cookie based.
If both domains are on the same server you could easily set this up with a vhost configuration in apache/nginx what ever you are using.