Plugin with specific admin menu icon

My new plugin uses the default icon set of WP in the admin-menu. That is not what I want.

How do I code within my plugin or via functions.php a specific admin menu icon that suites my needs for my plugin?

Is there a tiny code script / example to use where I can define what icon/image I want?

Also do not want to use an external plugin for this. It has to be a part of my plugin.

Hope some of you can give me a good start with this.
Thanks.

For custom menu you can use:

<?php 

    add_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function, $icon_url, $position ); 

?>

Second last option is for the custom icon url

Here is the reference from wordpress.org, you can read more about wordpress administration menu.