You want to change the sender e-mail address (From:) as well as the reply address (Reply-To:) in Microsoft Bookings.

When you create a Booking page in Microsoft bookings a scheduling mailbox is created.
Those mailboxes are not shown in Exchange admin console by default and you need to use Exchange Online PowerShell in order to modify them:

In the following screenshot you can see how to list boooking mailboxes, and how to set a new sender e-mail address

Normally it can take 10-15 minutes for the change to become effective.

The reply address can be changed via the Booking admin interfaces as follows:

Once you have modified the reply e-mail address, you need to re-publish your Bookings page as follows:

Dejan Foro