To add the search form through the widget, click on “Appearance -> Widgets” and add the Classic Search or Overlay search block. You can read more about the block options on this page.
If you are not using a block editor, instead, and need to use the legacy widget, select the YITH WooCommerce AJAX Search widget to the sidebar as shown in the image below.
The widget allows you to add a title and choose one of the Search shortcodes that you can create from the “Search Shortcodes” section in the plugin settings. For each shortcode, you will be able to customize the way the search form will be displayed.
Please, refer to this page for more details about how to create and customize a preset shortcode.