https://wa.me/<YOUR_WABA_PHONE_NUMBER>?text=<YOUR_START_MESSAGE>will take the user straight into WhatsApp (if it's installed) or to a landing page hosted by WhatsApp (if it's not installed).
whatsapp://r- account confirmation link - which doesn't seem to work on iOS
whatsapp://send- contact picker
http[s]://v.whatsapp.com- verify SMS deeplink
http[s]://api.whatsapp.com- text and direct chat deeplink
http[s]://wa.me- text and direct chat deeplink
http[s]://chat.whatsapp.com- accept invite link deeplink
wa_idto the phones-object when the user receiving a contact shall be able to save it to the phone.
wa_idit's only possible to "invite" users. If the
wa_id(=E.164-formatted version of the phone number) is given, the WhatsApp client will allow to "add a contact" and "send a message" which might come in different flavours on Android and iOS.