Add line item property when there is no liquid file

Hello I would like to add the following line item property to a single product collection so it only shows on those items pages, I have tried adding the following code to the main-product.liquid but that does not work and the template does not have its own liquid file only JSON and I am not sure how it could be added there, any help would be appreciated. Thank you!

{% form 'product', product %}
{%- if product.handle contains "subscription" -%}
 <p class="line-item-property__field">
  <label for="allergies">Allergies/Notes<label><br>
    <textarea required class="required" id="allergies" name="properties[allergies]"></textarea>
</p>
{% endif %}
{% endform %}

What do you mean by “line-item”? Thanks!

A input field line item property on a product page. (See example code) Thanks