Use simple-tags in frontend

Hi! First of all, Thank you for your works!
I’m trying to put an autocomplete box in my frontend post editing page..
I altered simple-tags.php to always load admin parts, commenting
/if ( is_admin() ) {

Then I put in my frontend template
metabox($thispost->ID);
that, in my mind, will put an autocomplete in my form.
But, I always get this error from apache
Call to undefined function metabox()
I also tried with

In the end, I will use it to memorize 1 category (and his parents)
instead of the standard category selection.I’ve already edited the plugin to search in category taxonomy instead of tags, and to automatically charge also parents category in my database.

I need it because I charged in categories all Italian Cities, and have a dropdown it’s not so beautiful.

I’m using frontier-post ..
What can I do ?
Thank you so much!
Davide

https://www.ads-software.com/plugins/simple-tags/