1- Firstly you need to claim your domain domain. To do so, check: Domain Insights
2- On the Layout Block of most Theme you will find a similar code to:

  {% if template == 'product' %}
  <meta property="fb:app_id" content="123123123123123" /> 
  <meta property="og:title" content="{{product.name}}" />
  <meta property="og:type" content="product" />
  <meta property="og:image" content="{{product.images.first}}" />
  <meta property="og:url" content="{{product.url}}" />
  <meta property="og:site_name" content="{{store.name}}" />
  <meta property="og:description" content="{{product.description | strip_html}}" />
  {% else %}
  <meta property="fb:app_id" content="123123123123123" /> 
  <meta property="og:title" content="{{store.name}}" />
  <meta property="og:type" content="website" />
  <meta property="og:image" content="{{store.logo}}" />
  <meta property="og:url" content="{{store.url}}" />
  <meta property="og:description" content="{{store.description | escape | strip_html}}" />
  {% endif %}

3- Add or change the property: fb:app_id. This represents the "owner" of this website Domain.