Inspect `wp-admin/network.php` and `wp-admin/includes/schema.php` files. You can create a condition based on **WP_INSTALLING_NETWORK**


Update:

    if (is_admin()){
    	/* First Step */
    	if (!defined( 'WP_ALLOW_MULTISITE')){
    			$current_wp_config = file_get_contents(ABSPATH . 'wp-config.php');
    			
    			$my_defines = "/* First we put WP_ALLOW_MULTISITE  */\r\n" .
    					"define('WP_ALLOW_MULTISITE', true); \r\n";
    					
    			$current_wp_config = str_replace("/* That's all, stop editing", $my_defines . "/* That's all, stop editing", $current_wp_config);		
    			file_put_contents(ABSPATH . 'wp-config.php', $current_wp_config);	
    	}	
    	/* Second Step */
    	if (!defined("WP_DEFINES_IMPORTED") && defined("WP_INSTALLING_NETWORK") && $_POST){
    		$current_wp_config = file_get_contents(ABSPATH . 'wp-config.php');
    		
    		$my_defines = "/* Now we define */\r\n" .
    			"define('WP_DEFINES_IMPORTED', true); \r\n" .
    			"define('MULTISITE', true); \r\n".
    			"define('SUBDOMAIN_INSTALL', true); \r\n".
    			"define('DOMAIN_CURRENT_SITE', '". get_clean_basedomain() . "'); \r\n".
    			"define('PATH_CURRENT_SITE', '". parse_url(  trailingslashit( get_option( 'home' ) ), PHP_URL_PATH ) . "'); \r\n".
    			"define('SITE_ID_CURRENT_SITE', 1); \r\n".
    			"define('BLOG_ID_CURRENT_SITE', 1); \r\n";
    		
    		$current_wp_config = str_replace("/* That's all, stop editing", $my_defines . "/* That's all, stop editing", $current_wp_config);		
    		file_put_contents(ABSPATH . 'wp-config.php', $current_wp_config);	
    	}
    }


 1. put this to mu-plugins folder.
 2. fresh wp install
 3. go to admin panel
 4. click to tools
 5. click to network setup
 6. run the setup
 7. ???
 8. profit!!!

![enter image description here][1]


  [1]: https://i.sstatic.net/6RVxp.png