Pinterest Addtocart Event Code - SENSE Theme

[Ilias_]

Hi all, 

This is about adding pinterest event codes to the SENSE theme. 

I want to add the addtocart code but don't have any product.liquid or similar files as mentioned in the help and here and there.  
I also searched for the custom.js in assets, following another tutorial but I don't have that file either. 

I have this in the Snippets > buy-buttons.liquid  code   ( the 'else' part in here):

<button
            id = "ProductSubmitButton-{{ section_id }}"
            type="submit"
            name="add"
            class="product-form__submit button button--full-width {% if block.settings.show_dynamic_checkout %}button--secondary{% else %}button--primary{% endif %}"
            {% if product.selected_or_first_available_variant.available == false or quantity_rule_soldout %}
              disabled
            {% endif %}
          >
            <span>
              {%- if product.selected_or_first_available_variant.available == false or quantity_rule_soldout -%}
                {{ 'products.product.sold_out' | t }}
              {%- else -%}
                {{ 'products.product.add_to_cart' | t }}
              {%- endif -%}
            </span>

And I also have this in the theme.liquid file : 

      window.variantStrings = {
        addToCart: `{{ 'products.product.add_to_cart' | t }}`,
        soldOut: `{{ 'products.product.sold_out' | t }}`,
        unavailable: `{{ 'products.product.unavailable' | t }}`,
        unavailable_with_option: `{{ 'products.product.value_unavailable' | t: option_value: '[value]' }}`,
      }

But I can't find the add_to_cart function.  So I don't quite know how and where I could add that code. 

Thanks in advance for helping me out!