Google Tag Manager / Ads conversion Issue

I'm trying to setup Google Ads conversions - conversions are based on users submitting an enquiry form on one of our tours (we have many tours & needs to be setup on all tour enquiry forms).

I can't add Google ad conversion code to the enquiry form so have been trying to setup Google Tag Manager which should trigger when a form is submitted. I can't get it to work for some reason (or it is working and I can't see it). I'm hoping it's an easy fix for someone as I haven't setup Tag Manager before.

An example tour is https://touringhighlights.com/tour/north-thailand/

Thanks!