@Clare0 Thanks. I got hold of @Airbnbhelp and they said that "unfortunately we don't have a setting that allows you to set a required check-in day of the week at this point. You can leave a note about this requirement in your listing description at this point."
Fom another posting says can do it if set each period in the 'Add another reuirement' box. but unless you put min/max at 7-days I cant see how it will work and then you limit guests to one week -as described in this post about 7, 14, 21 days. All a bit boring really as can't set up instant book otherwise could land up with orphan days.
Maybe you can ask how your neighbour set up the feature /workaround? Perhaps it is a just a note in the listing description.