How To Add Icon for WP Plugin’s Custom Menu Page

#Dashicon method

add_action( 'admin_menu', 'register_my_custom_menu_page' );
function register_my_custom_menu_page() {
  // add_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function, $icon_url, $position );
  add_menu_page( 'Custom Menu Page Title', 'Custom Menu Page', 'manage_options', 'custom.php', '', 'dashicons-welcome-widgets-menus', 90 );
}

#Normal Icon Method

/**
 * Register a custom menu page.
 */
function wpdocs_register_my_custom_menu_page() {
    add_menu_page(
        __( 'Custom Menu Title', 'textdomain' ),
        'custom menu',
        'manage_options',
        'myplugin/myplugin-admin.php',
        '',
        plugins_url( 'myplugin/images/icon.png' ),
        6
    );
}
add_action( 'admin_menu', 'wpdocs_register_my_custom_menu_page' );