If you need to show the add to cart button in a custom template file, or in a block where it does not come by default (for example, related products block), you should do the following:

• Get the corresponding phtml file.

Note: By default, Magento uses the same template for product related, upsell, crosssel and new products content widget.

This template comes from vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml.

If we want to custom this template, we should copy it to our current theme: app/design/frontend/{Vendor Theme}/{Theme name}/Magento_Catalog/templates/product/list/items.phtml.

• Add the following code:
  
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  <?php $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); $listBlock = $objectManager->get('\Magento\Catalog\Block\Product\ListProduct'); $addToCartUrl =  $listBlock->getAddToCartUrl($product); ?>   <form data-role="tocart-form" action="<?php echo $addToCartUrl; ?>" method="post">     <?php echo $block->getBlockHtml('formkey')?>     <div class="btn">         <button type="submit" title="Add to Cart" class="action tocart primary">             <span>Add to Cart</span>         </button>     </div>   </form>

  
  Bear in mind, that using the object manager is not a good practice, and you should avoid using it. If you have the chance to get the addToCart url by an item within the block you are using, or with a product object that you already have, it will be better.