Klaviyo:
Go to Audience > Lists & Segments
Create List / Segment
Select Segment
Give your Segment a name
Under Definition, select Properties about someone
The Dimension should be charles_opt_in_status is not set:
7. Select Create Segment
8. Then, use this list to send a Campaign encouraging these contacts to opt-in for WhatsApp.
For best results, incentivise with a special deal or discount!