Reading $_POST within WP-Admin

Hi Experts,

I am trying to write a very simple plugin for my client, and this is my first plugin, but cannot figure out how to submit from within /wp-admin?

As a test , I put together the following, but when I submit I get 404 Error. I see this is because the submit action is looking for .../wp-admin/admin-submit.php where admin-submit.php is in my plugins folder, but if I submit directly to it, I know I will get out of the wp-admin structure.  Do I need to doeverythign in iFrame or how do I handle this properly?

aces-test.php

<?php
/**
Plugin Name: Aces Test
Plugin URI: https://www.aces-project.com
Description: This is my <b>Plugin</b> description.
Version: 1.0.0
Author: Aleksandar Poposki
Author URI: https://www.aces-project.com
**/

function wp_add_menu(){
//    add_menu_page('ACES Page Title', 'ACES Options', 'manage_options', 'aces_plugin', '', 'dash-email-alt');
    $admin_page = 'admin-form';
    add_menu_page(
        'Custom Menu Title',
        'custom menu',
        'manage_options',
        $admin_page, 'aces_admin',
        '');
}

function aces_admin(){

    require_once 'admin-form.php';

}

function aces_activate(){
    //Create WP-Admin Page
    
    //echo 'ok';
}

// Activate
register_activation_hook(__FILE__, 'aces_activate');
add_action('admin_menu', 'wp_add_menu');   

function aces_shortcode(){
    $disp = "ACES <b> plugin </b> from nnethbeans.";
    return $disp;
}
add_shortcode('aces-plugin', 'aces_shortcode');
?>

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admin-form.php

<?php
/*echo '<div id="divAdmin" style="display:inline-block;  height: 500px; background-color: yellow">
    xxxxxxxx
</div>';*/
?>
<div id="divAdmin" style="display:inline-block;  height: 500px; background-color: yellow">
    <form action="admin-submit.php" method="post">
        name: <input type="text" name="txtName">
        <input type="submit">
    </form>
</div>

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admin-submit.php

<?php

$name = $_POST['txtName'];
echo 'Are you sure your name is ' . $name . '?';

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Any help will be greatly appreciated.