Add External Buy Button with Metafields to Product Page - Justin Hayward

1. Open product.liquid and Locate {% if has_cart %}{% include 'product-add-to-cart-form' %}{% endif %}
2. Replace the {% if has_cart %}{% include 'product-add-to-cart-form' %}{% endif %} with the following code

      {% if product.metafields.external-url != blank %}
        <a href="{{ product.metafields.external-url.external-url }}" target="_blank" id="purchase" class="addtocart  btn-main purchase-btn btn-primary">
          PURCHASE
        </a><br />
      {% else %}
        {% if has_cart %}{% include 'product-add-to-cart-form' %}{% endif %}
      {% endif %} 

1. Now anytime a product uses the metafields info and uses "external-url" for both the key and namespace, the external button will show instead of the standard add to cart button.

Reference: https://justin-hayward-2.myshopify.com/