On the Settings page, you will see a section dedicated 'WhatsApp Numbers / Agents'.
The title section of this area contains a blue button on the right called 'Create Number / Agent'.
Once clicked, it will add a new number / agent to the list for you.
This new number will be inactive by default and have no predefined GEO country.
From here you can click the settings cog icon at the end right of the row to edit. See below for editing the number / agent
