Option 1: Enter it manually
Open Calendar, tap +, and fill in the title, date, start time, and end time. It's reliable but slow when you do it many times a day — and you still have to switch back and type the reply.
Option 2: Use iOS data detectors in Messages
When Messages recognizes a date or time, it underlines it. Tap it and choose Create Event. This works for clear phrases in Apple Messages, but it doesn't know your appointment lengths, doesn't check conflicts, and doesn't help with Instagram or WhatsApp DMs.
Option 3: Use a message-to-calendar app
Copy the message (or screenshot a DM) and hand it to an app built for this. BookingSolved reads it on your iPhone, flags anything ambiguous like “next Friday”, checks your Apple Calendar, and adds the appointment only when you tap to approve — with a reply ready to send.