Wishlist Hero adds a snippet during installation that you can use for this purpose. Instruction for most themes are:

  1. On you store admin screen, Go to “Online store” > “Themes
  2. Check the theme you want to edit, and click on the “Actions” button, then on “Edit Code
  3. Now you are on the Theme files editor, you should look for where you want to place the wishlist icon in header. In most cases that is “header.liquid
  4. Start editing the file, search for the cart code. right before the cart please place the following snippet.

    {% render 'wishlisthero-header-icon.liquid' %}
  5. Save the file and preview in store.
  6. Optional: If you want to edit how the icon looks, you can always add any styles you want to add in “snippets/wishlisthero-styles.liquid