In most cases we will see the first option, which is also the easiest to track. But in other cases we will see the second option, which, although it is more complex to configure, also has a very interesting advantage.
Advantage of having a message pop up when a user fills out the form
Mainly, we will be able to analyze on which page the taiwan cell phone number conversion has been made . And this is something interesting when we have forms on many pages, as happens in the education sector , where each course has a page with a form.

If, on the other hand, we direct the user to a thank-you page , we will see in Google Analytics 4 that all conversions occur on this page. Without giving us a clue as to which pages have received the most conversions.
How to track this message
Let's see what trigger we need to create to do the tracking, since this same trigger can be used for a GA4 event, a Google Ads conversion or a Social Ads event .
While to track a thank-you page we use Page Display, to track a message that appears on the screen we need to use the trigger: Element Visibility .
Element visibility GTM
Element Visibility Trigger Settings
We are now editing this new trigger. Now we need to know what code appears when we fill out the form. To do this, we would need to run a test by filling out a form, and then inspect the code of the message that appears.