Once you have created a new relationship, you can initiate a call or message using Whatsapp.

Once you choose a relationship, press on Get in Touch and choose the WhatsApp option to call or text them.

Note: You need to have your relationship in your contact list to be able to use WhatsApp, or else WhatsApp will ask you to invite them to join their app. 

In order for WhatsApp to understand the number, that might require the full international format (e.g. +306979112902).