If you don’t want to give Whatsapp access to your contacts, you can use the click to chat API to start a new chat by simply proving the number (in this case 49123456789):
https://wa.me/49123456789
It’s also possible to prefill the chat with text:
https://wa.me/49123456789?text=I'm%20interested%20in%20your%20car%20for%20sale