Show messages in a pop-up?

How can I show the messages of “successful sending” and “validation errors ocurred” in a pop-up box? I need it because placing it in any other place would ruin the page design.

I’ve tried putting this on the additional settings box: on_sent_ok: "alert('Your message here!');" but it doesn’t do anything at all. I also get no errors on the javascript console.

If anyone can give me a hint on this, here’s the link: https://inpabol.com/contato-2/