custom-page-in-user-accounts-panel.php

<?php
/**
 * Insert the new endpoint into the My Account menu.
 *
 * @param array $items
 * @return array
 */
function ci_custom_account_menu_items( $items ) {
    // Remove the logout menu item.
    $logout = $items['customer-logout'];
    unset( $items['customer-logout'] );

    // Insert your custom endpoint.
    $items['ci-custom-endpoint'] = __( 'CI Custom Endpoint', 'woocommerce' );

    // Insert back the logout item.
    $items['customer-logout'] = $logout;

    return $items;
}

add_filter( 'woocommerce_account_menu_items', 'ci_custom_account_menu_items' );


function flush_rules(){
  add_rewrite_endpoint( 'ci-custom-endpoint', EP_ROOT | EP_PAGES );
    flush_rewrite_rules();
}
add_action('init','flush_rules');


/**
 * Endpoint HTML content.
 */
function ci_custom_endpoint_content() {
    echo '<p>Hello World!</p>';
     
    //add below file within theme directory
    include get_template_directory().'/my-custom-endpoint.php';
    
}

add_action( 'woocommerce_account_ci-custom-endpoint_endpoint', 'ci_custom_endpoint_content' );