Start with a new post or edit an existing post that you want to add a ShopStyle (or similar widget) to.
Enable Custom Fields in the post editor
In the post editor, click on the 3 vertical dots menu icon in the upper right of the screen and select Options. Under Advanced panels make sure Custom fields is checked. If a button appears that says Enable & Reload, click that. Now you will have a Custom Fields section below the post editor.
Add your widget code
In the box under Name, type shop In the box under Value, paste your code.
Click the Add Custom Field button.
Publish or Update your post.