On the Ads tab, select the WhatsApp ads, open Bulk edits, click Edit Ad, and choose WhatsApp. You set the Welcome Message, which is the text people see when they open the chat, and the Ready Message, which is already typed in for the person to send. The welcome message is required. If you often use the same texts, click Save template, then pick the saved template from the list next time instead of typing again. The same editor appears when you edit a single ad with the pencil icon on its row, if the ad is a WhatsApp ad.