place a small "static ad" over the top of the slider

you can place a container as you like – best place on a fullwidth slider might be the avia-slideshow-inner container:

style a div as you like to have. Copy that html code.
clean up the html code to remove spaces between tags (inside a tag you can have it)

for better select i gave the slideshow alb element a custom-class: ads-in-slider

here is an example page: https://webers-testseite.de/bemodesign/
here is the jQuery Code for it – put it in child-theme functions.php:

function insert_to_a_fullwidth_slider() { 
if(is_page(37941)){
?>
<script>
(function($){
	$( '.avia-fullwidth-slider.ads-in-slider .avia-slideshow-inner' ).append('<div class="add-in-slider" style="background-color: #21495d; padding:0 20px 10px 20px; border-radius:0px 0px 30px 0px ; "><section class="av_textblock_section " itemscope="itemscope" itemtype="https://schema.org/CreativeWork"><div class="avia_textblock  av_inherit_color " style="color:#ffffff; " itemprop="text"><p><img class="alignnone" src="https://webers-testseite.de/wp-content/uploads/rhino-on-blob.png" alt="rhino-logo" width="150"></p><div class="hr hr-short hr-left" style="margin: 10px 0;"><span class="hr-inner"><span class="hr-inner-style"></span></span></div><p>Skip your<br>security deposit<br>with <a href="https://guenterweber.com" target="_blank" rel="noopener noreferrer">Rhino</a></p></div></section></div>');
})(jQuery);
</script>
<?php
}
}
add_action('wp_footer', 'insert_to_a_fullwidth_slider');

i gave a custom-class to the inserted div too : add-in-slider
sorry sounds equal but isn’t – change it to your needs.

then some options for the inserted div in quick css:

.page-id-37941 .add-in-slider {
    position: absolute;
    border: 1px solid #fff;
    display: inline-block;
    z-index: 10;
    top: -1px;
    left: -1px;
    box-shadow: 3px 3px 20px #fff;
}

I would strongly recommend to install a child theme for Enfold.
You could of course place it in the functions.php of the parent theme, but these entries would be lost with the next update.
So you have exactly one place for such snippets – the child-theme functions.php.

See here some info and donwload a predefined child-theme: https://kriesi.at/documentation/enfold/child-theme/

I do that from the beginning of all my installations – but Enfold offers some tools to take over the parent-theme settings.
On Enfold – Import/Export there are some buttons that could help you.

My experience and the reading here on board show that the following procedure is probably the most successful:

• Parent Theme active : export theme settings file
• Child Theme activate then : import that file.

for responsive case maybe you decide to shrink a bit or to remove parts of the content:

@media only screen and (max-width: 767px) {
  .page-id-37941 .add-in-slider {
    transform-origin: left top;
    transform: scale(0.6);
    transition: all 0.7s ease
  }
}

@media only screen and (max-width: 480px) {
  .page-id-37941 .add-in-slider img,
  .page-id-37941 .add-in-slider .hr-short{
    display: none
  }
}

As Rikard said – you had to adjust it to your page-id

if you like to have a big area as aktive link you can surround inside that div the section which is all arround the content (to surround the whole div i would not try) – because an anchor as a block-level element is hard to style f.e. with border-radius and it is hard to position in that manner so my solution now on that page this way:

<div class="my-styling-for-that-ads" …><a href="https: …><section>your content </section></a></div>
and give the padding to the section or to the textblock as here in the code:

function insert_to_a_fullwidth_slider() { 
?>
<script>
(function($){
	$( '.avia-fullwidth-slider.my-fullwidth-slider-with-that-ads .avia-slideshow-inner' ).append('<div class="my-styling-for-that-ads" style="background-color: #21495d; border-radius:0px 0px 30px 0px"><a href="https://YOUR-DOMAIN.com" target="_blank" rel="noopener noreferrer"><section class="av_textblock_section" itemscope="itemscope" itemtype="https://schema.org/CreativeWork"><div class="avia_textblock  av_inherit_color " style="padding:0 20px 10px 20px; color:#FFF " itemprop="text"><p><img class="alignnone" src="/wp-content/uploads/rhino-on-blob.png" alt="rhino-logo" width="150"></p><div class="hr hr-short hr-left" style="margin: 10px 0;"><span class="hr-inner"><span class="hr-inner-style"></span></span></div><p>Skip your<br>security deposit<br>with <strong>Rhino</strong></p></div></section></a></div>');
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'insert_to_a_fullwidth_slider');

just for a better understanding the inserted div – see a solo html page with that div only and the tree-structure:

but we need that div in a one-liner to insert it with append in the functions.php. Otherwise we will have a php error on that.

PS : you then can have too a hover style for the whole div:

.my-styling-for-that-ads:hover {
    background-color: #0040ff !important;
}

All opening brackets need a closing one!
so if you get rid of: if(is_page(19)){

you had to find the corresponding closing curly bracket. -( the round one are neutral the open and just closes a few letters behind)

that is the reason why your code is not without error.

Another hint: here on board it might be usefull to insert code as code – therefor we had here the code tag above.
First click on code – then insert your code – click now on /code (oben a tag – close a tag as always)

To look what might be not correct it is best to have it 1/1 – boardsoft changes if you do not use the code tag the content ( f.e. quotation marks to something different etc. pp )