Here i'm going to share some nice feature in Nintex form to show the confirmation message, once the item added in the List.
Activate the Nintex forms for List at the site collection level, then create a Image file with nice colorful and with your text and place the same in any document library or picture library. This image file will show once the users submitted the item in the list.
Following to this go to Nintex form settings page, put the image URL in the "Redirect URL" under Advanced section. Click save and publish the Nintex Forms. Now try to added a new item to the list and the colorful image pop after submitting the item.
In addition, we can use Submit button option to show the confirmation message. Control Settings of Submit button, under advance section provide the message in the "Confirmation message" box.