This is the schema you need to use if you wish to mention someone on a whatsapp chat (also valid for whatsapp groups):

{ "number": "GROUP_ID_OR_PHONE", "msg": { "text": "We are mentioning @TLF_NUMBER on this line!", "mentions": ["TLF_NUMBER@s.whatsapp.net"] } }

where:
TLF_NUMBER is a valid mobile phone you wish to mention (including country code).
GROUP_ID_OR_PHONE is the group-id of the whatsapp group you are sending the message to. If a chat, then the recipient must be the number you want to mention, for obvious reasons.