Adding short code

Good evening,
I am trying to create the new site where to put the sync code, but as I click to add page a new window with automatic preset theme appears. How can I add the page so that it gave me the frame where to put the [easync_hotel_code] as in tutorial (https://www.youtube.com/watch?v=zM5wPe8NKic&t=179s&ab_channel=SyntacticsInc.)? Or can I insert the sync code somewhere else (when editing https://salernoexperiencehostel.com/booking-page/)? Or do I have to change the theme/theme setting? Thank you in advance for any advices!

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