Shopify Form Submission - event does not appear

I have a shopify POWR form and I followed this step to install tracking, but it didn't work:
https://help.powr.io/hc/en-us/articles/115002720987-Steps-to-Add-a-Google-Analytics-Tracking-Code-to-POWR-Forms

Not sure if there is another way to tracking this because no event appears when I submit a form. Nothing happens. I did Inspect -> element, and can see that this form is in an iFrame.

I was wondering if anyone has been able to resolve this?

Thanks!