Reserving Meetings in Google Meet

Any time you create an event in Google Calendar there is a button on "Joining Info" with a link to "Add video meeting details" if you click that is will automatically reserve that slot in Google Meet for you.  You can do this well in advance of the actual date and get a link for the meeting ahead of time.

When you go back and view the event you will see the link to the Google Meet that you can share with others.

What if you already scheduled a meeting?  

Go back into the Google Calendar invite and click to edit it.  Then, under "Event Details" the second item on the list should be a dropdown that says "Add Conferencing".  Click to add the Hangouts Meet to the event.  A code will then be generate for the event that you can share.

If you want more info please review the Google Meet Guide for details.