Add query arg for pretty permalinks in wordpress way

Question

Currently I am using this-

add_rewrite_endpoint( 'download', EP_PERMALINK | EP_PAGES );

add_filter( 'query_vars', 'add_query_vars');
function add_query_vars($vars){
    $vars[] = "download";
    return $vars;
}

Then this to generate url-

global $wp;
$current_url = home_url(add_query_arg(array(),$wp->request));
$book_download_url = $current_url."/download/{$value}";

Both the parts are working fine and give-

...post_url/download/42xcd

But I want to do the second part in wordpress way.

add_query_arg gives

...post_url?download=42xcd

from code-

$__new_url = add_query_arg('download','42xcd');

while I want to have

...post_url/download/42xcd

What am I missing here? Or any other approach? I tried set_query_var too.