[Plugin: TDO Mini Forms] Using Append

I want to use Append to add a field name to also be a tag, but can’t seem to work out the right code.

Basically I have a custom field in my list of inputs that I would also like to use as a tag.

The field name is currently “TDOMF Form #5 Custom Field #_7”

Using Append to post, I would like to duplicate the result of that entry as a tag.

I have assumed that I would use something like this, but need some final help to correct it.

Append:
<?php
$post_code = get_post_meta($post_id, 'TDOMF Form #5 Custom Field #_7', true);
$post_tag .= $post_code;
$postdata = array("tag"=> $post_id, "post_tag" => $post_tag);
sanitize_post($postdata,"db");
wp_update_post($postdata);
?>

If anyone can help, this would be useful to me. Thanks.