There's no official "must use for 30 days" policy in the public documents. This message is Meta's way of stating that your phone number's quality rating is too low for the messaging limits required for advertising.
Quality Rating: New WhatsApp Business numbers start with very low messaging limits (sometimes only 250 unique delivered messages in a 24-hour period).
The System's Logic: Advertising (especially large campaigns) requires a high messaging limit. The system is delaying the full link until your number is active and has proven it won't send spam.
The Fix: You have to prove you are a legitimate, active user.
Complete Your WhatsApp Profile: Ensure your profile on the WhatsApp Business app is 100% complete: name, category, hours, address, and a good profile picture.
Actively Use the Number: For the next few days, send and receive non-spammy messages with a few different contacts. You need to show Meta you are using the app for genuine customer communication.
Monitor Your Messaging Limit: You can actually check your messaging limit in WhatsApp Manager (found within Business Settings). Once you successfully send a few messages and your quality rating improves, your limit should automatically increase, which is the signal Meta needs to unlock the full advertising features.