WhatsApp messaging runs through the WhatsApp Business platform via Twilio, so you send from an approved business number rather than a personal account.
Go to Settings → WhatsApp → Numbers to add and manage your sending number(s). Once a number is connected, it becomes available as the sender when you create a WhatsApp broadcast — or a WhatsApp step inside a sequence.
SMS also uses your Twilio numbers, managed in the same WhatsApp settings area. Connect the numbers you'll send from before building a broadcast, so they're ready to pick in the wizard.