Just chat and in between of chat, ask her that ‘Are you on Whatsapp ?’ (By pretending that you are about to leave and cannot continue the talk). Then say okay, let’s continue on Whatsapp.

Always leave on an interesting note and leave. There should be something left to continue.

https://hemantpandeyblog820899289.wordpress.com/2017/09/16/when-do-i-know-when-to-propose/