How to add just one specific page to widget without plugin?

Follow this step for Only one page add custom sidebar

Step 1: Put this code in your theme function.php file. Register custom sidebar and your custom add_action.

function wpdocs_theme_slug_widgets_init() {
    register_sidebar( array(
        'name'          => __( 'Custom Sidebar', 'textdomain' ),
        'id'            => 'sidebar-10',
        'description'   => __( 'Widgets in this area will be shown on all posts and pages.', 'textdomain' ),
        'before_widget' => '<li id="%1$s" class="widget %2$s">',
        'after_widget'  => '</li>',
        'before_title'  => '<h2 class="widgettitle">',
        'after_title'   => '</h2>',
    ) );
}
add_action( 'widgets_init', 'wpdocs_theme_slug_widgets_init' );

add_action( 'your_custom_hook_name', 'your_function_name' );
function your_function_name( $sidebar_name )
{
    is_page( 'sample-page' ) && dynamic_sidebar( 'sidebar-10' ); // your sidebar name
    remove_action( current_filter(), __FUNCTION__ );
}

Step 2: Put this code where you display this widgets

<?php do_action( 'your_custom_hook_name' ); ?>

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