I need help in seeting up google tag manager to track form submitting (contact owner) on listing details page.

I have added custom form ID="kontaktiraj_vlasnika"
Code:
Only registered members can view the code.
, so that we can track form submission (contact owner) on listing details page.

Everything works fine, but problem it will track all form submissions, even those where form validation is not passed.

So question is, how to setup this in google tag manager to track only event form submission after successful form validation.

I can see that for is using
Code:
Only registered members can view the code.
Attached is screenshot from google tag manager,
Snimak ekrana (340)_LI.jpg