How I Can Use The Get Value as A Slug

You don’t need to do anything with rewrite rules.

Your example (“domain.com/example-slug/?holder=value”) is a perfect example of “Plain” Permalinks. So your starting point is to set Permalinks (Settings > Permalinks) to something other than “Plain”. This will take the “/?” component out of the URL, and generate a URL in your preferred format.

The second thing is to capture the URL, parse it and get the “value” (to use your words). You can do this with a function like the one below. Put this function in your functions.php (or a plugin if you prefer). Then call it on the page/post where you want to get the “value”. The function will yield the “value” as a variable $myvalue. You can expand the function to do more stuff, or you could return the value elsewhere to do something with it.

function getslug(){

    // get the url
    $myurl = home_url( $wp->request );
    //echo "<p>The page url is ".$myurl." </p>";  //DEBUG

    // Explode the url to get access to the taxonomy and terms
    $pageterms = explode("https://wordpress.stackexchange.com/", $myurl);
    //echo "the exploded components of the URL <pre>";print_r($pageterms);echo "</pre>"; //DEBUG

    // the value is always the last element in the url, so we'll count the number of elements
    $mycount = count($pageterms);
    //echo "<p>The element count is ".$mycount." </p>";  //DEBUG

    // since array keys always start with zero, the key for the value will be the count minus 1 (to allow for the zero)
    $myvalue = $pageterms[$mycount-1];
    //echo "<p>My value is  ".$myvalue." </p>";  //DEBUG

}

PS: I’ve left my echo statements (used for Debugging) in this example; you can just delete them if you don’t need/want them.