Follow Below steps to add "Add to Cart" button on collection page.
- Go to theme -> Action -> Edit Code -> Snippets -> product-thumbnail.liquid
- Paste below code at the line mentioned in the video.
- If you wanted to hide quantity input box then replace type=”number” with type=”hidden”
<form method="post" action="/cart/add">
<input type="hidden" name="id" value="{{ product.variants.first.id }}" />
<input min="1" type="hidden" id="quantity" name="quantity" value="1"/>
<input type="submit" value="Add to cart" class="btn" />
</form> 