在为meta field SEO Yoast管理面板创建的自定义变量的预览中显示内容

Question

I want to show post description as meta description on frontend.For this i created custom variable that will show post desction on frontend.

Here is the code which i am using to create custom variable and is working fine:

function retrieve_var1_replacement( $var1 ) {
    global $post;
    return strip_tags(substr($post->post_content, 0, 150));
}
function register_my_plugin_extra_replacements() {
    wpseo_register_var_replacement( '%%mycustomdesc%%', 'retrieve_var1_replacement', 'advanced', 'This is to show post description' );
}
add_action( 'wpseo_register_extra_replacements', 'register_my_plugin_extra_replacements' );

This code working fine on frontend but the issue is that on post page in admin panel, instead of showing description, its show %%mycustomdesc%% in the snippet preview box. It should show the same content that is coming on frontend.

Here is the backend screenshot for better understanding

Anyone can suggest me how i can show description in place of

%%mycustomdesc%%