Hey @Ebtihal the UTM will pass a source to the tracking cookie when a visitor clicks this. If the visitor is created from a form submission on a tracked page after first visiting the site through this link, it will populate the source property.
It sounds like you’re creating these via the contact creation api is this the case? IF so this will not pass a tracking cookie and thus will not have source tracking attribution on it. You would need to create the contact via a HS form or via the forms api. Without the tracking cookie there would be no way to track the source of the contacts. If they’re created prior to submitting a form with a tracking cookie, then it wouldn’t update the source if this happened after they were first created.
Depending on your api, it might also be worthwhile to check if you can pass a custom property for source to the contact record, something to denote that it is a whatsapp source which you could use to create a list or reports off of going forward.