Custom model - Position in Menu

Hi,

It works like the sub-menu page in WordPress

The slug name for the parent menu (or the file name of a standard WordPress admin page)

For example, if you want to show the transaction under the Page menu, assign the setting parent to this slug edit.php?post_type=page

function prefix_register_transaction_model() {
    mb_register_model( 'transaction', [
        'table'  => 'transactions',
        'labels' => [
            'name'          => 'Transactions',
            'singular_name' => 'Transaction',
        ],
        'menu_icon' => 'dashicons-money-alt',
        'parent' => 'edit.php?post_type=page'
    ] );
}

Get more details on the documentation
https://developer.wordpress.org/reference/functions/add_submenu_page/#user-contributed-notes