In order to have website start / popup chat buttons, you can use third party tools like

• elfsight.com
• https://www.futy.io/
• https://getbutton.io/
• https://premio.io/downloads/chaty
• https://codecanyon.net/item/walink-whatsapp-shortlink-and-floater-generator/48420348

You can integrate them in Settings→Custom JS → FrontEnd JS