Magento 2 – Customize Related Products Template

By default Magento, there is no setting option in Admin for Add to Cart button on the related products block. If we want to custom the related, upsell, crosssel and new product template, we should override this template vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml.

Change $showCart = true; in case - related product.

We can refer here: Related Products template customize

Replace the content of

if ($showCart) ...... endif

with below code.

<?php if ($showCart): ?>

<div class="product actions product-item-actions">
    <form  data-role="tocart-form" action="<?php echo $block->getAddToCartUrl($_item); ?>" method="post">
    <div style="float: left;">
        <input type="number"
               name="qty"
               id="qty"
               style="width:50px;"
               maxlength="12"
               value="<?php /* @escapeNotVerified */ echo $block->getProductDefaultQty() * 1 ?>"
               title="<?php /* @escapeNotVerified */ echo __('Qty') ?>" class="input-text qty"
               data-validate="<?php echo $block->escapeHtml(json_encode($block->getQuantityValidators())) ?>"
               />

        <?php echo $block->getBlockHtml('formkey'); ?>

        <button type="submit" title="<?php echo $block->escapeHtml(__('Add to Cart')); ?>" class="action tocart primary" style="float:right;margin-left: 10px;">
            <span><?php /* @escapeNotVerified */echo __('Add to Cart'); ?></span>
        </button>
    </div>
    </form>
</div>

It worked for me in magento version 2.1.3. Source post: How to add multiple products to a cart in related products grid on product detail page using Qty Box?

Hope it may help someone.