Despite being a rare use case, you may want to send a calendar invite as a part of your email communication. There are two ways to do it.
Create a separate link for Outlook, Gmail, Yahoo and Apple mails, and personalize the email link based on the customer's email addresses.
You can create the links using a tool like this one: https://www.labnol.org/apps/calendar.html
You can make an iCalendar file, which can be used across the different clients, but the user needs to download and open the event file, then add it to their calendars.
You can generate the calendar event in your own mail client and export it, or use a website like this one: https://ical.marudot.com/
Once you have the iCalendar file, upload it to google Drive, make it publicly accessible and add the link to your email's CTA. Here's how it looks when opened from Gmail on a Mac: