Identifier of the contact. Supports a contact ID, a phone number in E.164 format starting with + (for example, +16315551111), or a Meta username without the leading @.
A Meta username must contain 3 to 35 letters, digits, periods, or underscores. If a numeric value can be interpreted as both a Meta username and a contact ID, it is resolved as a Meta username first. If no contact has that Meta username, the value is resolved as a contact ID.