You can add code in end file addtocart.phtml.

vendor\magento\module-catalog\view\frontend\templates\product\view\addtocart.phtml

<!-- AddThis Button BEGIN -->
<div class="addthis_toolbox addthis_default_style addthis_32x32_style">
<a class="addthis_button_preferred_1"></a>
<a class="addthis_button_preferred_2"></a>
<a class="addthis_button_preferred_3"></a>
<a class="addthis_button_preferred_4"></a>
<a class="addthis_button_compact"></a>
<a class="addthis_counter addthis_bubble_style"></a>
</div>
<script type="text/javascript">var addthis_config = {"data_track_addressbar":true};</script>
<script type="text/javascript" src="//s7.addthis.com/js/300/addthis_widget.js#pubid=ra-51a5fa9c59636a3d"></script>
<!-- AddThis Button END -->