Magento: Add to Cart Url in any page

$product_Id = '100';
$_product = Mage::getModel('catalog/product')->load($product_Id);
$url = Mage::helper('checkout/cart')->getAddUrl($_product);

Or

<button class="button btn-cart"title="Add to Cart" type="button" onclick="window.location.href='<?php echo Mage::helper('checkout/cart')->getAddUrl($product);?>/?qty=1'">
      <span>
          <span>Add to Cart</span>
      </span>
</button>