Magento 2: Add to cart button on phtml file inside Magento

Question

I have a Magento 2.2.9 based website and a cms page created that called in a phtml file.

In that phtml file, I am wanting to have an add to cart button for a specific product. However, clicking the button does not seem to add the item to my cart. I am wondering if anyone could suggest changes tome code get it to work.

        <?php 
            $sku ='KACI01';
            $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); 
            $productRepository = $objectManager->get('\Magento\Catalog\Model\ProductRepository');
            $product = $productRepository->get($sku);
            $productid = $product->getId();
            $cartHelper = $this->helper('Magento\Checkout\Helper\Cart');
            $addToCartUrl = $cartHelper->getAddUrl($product);

            $postDataHelper = $this->helper('Magento\Framework\Data\Helper\PostHelper');
            $postData = $postDataHelper->getPostData($block->getAddToCartUrl($product), ['product' => $productid])
        ?>

    <form data-role="tocart-form" action="<?php echo $addToCartUrl; ?>" method="post"> 
       <?php echo $block->getBlockHtml('formkey')?>
       <button class="action tocart primary" data-post='<?php echo $postData; ?>' type="button" title="Add to Bag">
          <span>Add to Bag</span>
       </button>
    </form>

Answer 1

I've updated your code. Please give a try with below code in your phtml file.

<?php 
    $sku ='KACI01';
    $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); 
    $productRepository = $objectManager->get('\Magento\Catalog\Model\ProductRepository');
    $product = $productRepository->get($sku);
    $productid = $product->getId();
    //$cartHelper = $this->helper('Magento\Checkout\Helper\Cart');
    //$addToCartUrl = $cartHelper->getAddUrl($product);
    $listBlock = $objectManager->get('\Magento\Catalog\Block\Product\ListProduct');
    $addToCartUrl =  $listBlock->getAddToCartUrl($product);

    $postDataHelper = $this->helper('Magento\Framework\Data\Helper\PostHelper');
    $postData = $postDataHelper->getPostData($block->getAddToCartUrl($product), ['product' => $productid]);
?>

<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>

Reference: https://www.offset101.com/magento-2-get-add-cart-button-custom-templete-file/

Hope it helps!!!