Hi Chris, you need to take 3 steps, I think:

#1 Install the free relevanssi plugin (you might need to enable the indexing of excerpts for this to work properly, I don’t know why?).
#2 Edit the child theme functions.php file and add this code:

add_filter('relevanssi_content_to_index', 'rlv_index_post_id', 10, 2);
function rlv_index_post_id($content, $post) {
    $content .= " " . $post->ID;
    return $content;
}

[If you are not using a child theme, you should. It is easy to make one. If you are unsure, and your theme doesn’t have a child theme making function, you can use the https://wordpress.org/plugins/orbisius-child-theme-creator/ plugin.]

#3 In the Form settings, under the Advanced tab, enable to Relevanssi support (you probably won’t need the relevancy setting though).

Then, any number you enter in the search box should be searchable.

I do not know if not using a child theme would cause this. Without this code, is everything working (apart from the POST ID search)?

FWIW, working without a child theme is not a recommended way to work. It is generally ‘a good thing’ to always have a child theme. They are easy to make. There is a good plugin you can use to do it (you can remove the plugin after you are done making it):

https://wordpress.org/plugins/orbisius-child-theme-creator/

Most odd. The only other way I can think to do this is a bit laborious, but it would be to add a custom field to the posts and copy the post id into that, and then in Relevanssi you can get it to index the custom fields.

I will have to ask Ross, the developer, for advice. I will get back to you.