Set business username

Claim or transfer a WhatsApp Business username for the account.

Username rules: 3-35 characters, letters/digits/period/underscore only, must contain at least one letter, no leading or trailing periods, no consecutive periods, no www prefix, no domain TLD suffix (e.g. .com).

If the desired username is currently held by another account, pass transferAction: "force_transfer" to request a transfer. On failure the API returns a standard error envelope with one of these codes: whatsapp_username_unavailable (already taken and transfer not requested), whatsapp_username_ineligible (account not eligible to claim a username), or whatsapp_username_transfer_required (username is held elsewhere; retry with force_transfer).