Send WhatsApp anonymously?

Not anonymously. You need an alternative number to run on WhatsApp (either with another phone, an Android emulator or just on your phone by uninstalling WhatsApp).

There are so-called temporary phone numbers in theory, with which you can register on pages and read out the incoming SMS, that is to say you can also receive the code to register (use Google once). I don’t know if these numbers work with WhatsApp. Many providers recognize such numbers and have blocked them.

Otherwise, you could also register by landline, as if you don’t fool me, instead of texting you can also get a call from WhatsApp with the registration code.

Alternatively, you could also buy a virtual number and use it via Skype. Let’s get on the same way.

Finally, I remember a prepaid card from Aldi.

If anything would be worth it, you can only decide. But you can’t send an anonymous message from your now number.