• Step 1: make your own function inside your plugin
• Step 2: call add_shortcode inside your plugin
• Step 3: use the shortcode in a text widget

Step 1: make your own function inside your plugin

/**
 * shortcode: site url
 * @return string
 */
function wp_home_shortcode() {
    return WP_HOME;
}

Step 2: call add_shortcode inside your plugin

add_shortcode('wp_home','wp_home_shortcode');

Step 3: use the shortcode in a text widget

Just add a Text widget from WordPress administration and type [wp_home] inside the box to get the current url of your WordPress installation