Getting started with custom widget

Hello, I am following your Creating a widget tutorial. To create a widget it is needed to input the following code into some file.

<?php
function add_my_awesome_widgets_collection($folders){
    $folders[] = 'path/to/my/widgets/';
    return $folders;
}
add_filter('siteorigin_widgets_widget_folders', 'add_my_awesome_widgets_collection');

Which is the file to put the code is not mentioned, so I’m not sure.
For a case, here is my file tree for SiteOrigin Widget Bundle folder

Where to put the code exactly?
Thanks in advance!