A common approach for custom integrations like this is to place your conversion script on the Thank You page, since that page is only reached after a successful order.
For passing purchase data, you may want to take a look at this discussion:
https://community.funnelish.com/t/exposing-revenue-value-via-datalayer-push-on-thank-you-page-for-taboola-gtm-tracking/9356/6?u=anwerashif
In that thread, we discussed how some order-related data is already available on the Thank You page. If you inspect the browser console, you’ll find the Funnelish payment/order object loaded there. Your developer can use that data to:
- Read the purchase amount
- Read customer/order information
- Push it into a dataLayer
- Fire custom tracking scripts such as OpenAI Ads conversion events
So the general flow would be:
- Add the OpenAI conversion script on the Thank You page.
- Read the order value from the available Funnelish order/payment object.
- Pass that value into your OpenAI conversion event.
- Test with a real or test purchase to verify the event fires correctly.
At the moment, I am not aware of any Funnelish-specific documentation for OpenAI Ads tracking, so this would fall under a custom integration. If you have a developer helping with the implementation, the thread above should give them a good starting point 
If you get stuck, feel free to share more details about the OpenAI conversion script requirements.