Hi there,
You can use the Advanced Custom Fields plugin to add a custom field to the Widget Blocks. To show this post on (the public section / frontend) of your website you will need some additional code though.
You can use the ww_content
filter hook to add the value of this custom field to the widget content. Are you familiar with PHP and the way WordPress filters work? Your code would look something like this.
function my_widget_block_content($content, $id) {
$field = get_field('my_custom_field', $id);
$content .= "\nField value: {$field}";
return $content;
}
add_filter('ww_content', 'my_widget_block_content', 10, 2);
Hope that helps!