Why is the assistant asking for OTP or agenda, or refusing to let me book on a certain day?

A good assistant will always follow the host’s preferences. If you are asked to enter an agenda or an OTP, it’s because the host has enabled that option. An OTP is typically required when the host’s link is fairly accessible, such as when it’s included in an email signature, and it serves as a safety feature to prevent abuse. A host can also set other restrictions, such as how far in advance an appointment can be scheduled and how much notice must be given.