Hello @Gaelle52,
I'm not sure how the website behaves in this regard for hosts in different areas. For me, setting a minimum number of nights for a specific period is the same as it alway was.
Go to your calendar and click on "Availability". Then scroll down to the "Trip length" section and click on its "Edit" button. Then click on the "Add a custom rule for seasons or specific dates" link. Click in the "During" box and select the "Specific dates" option and enter the start date, end date and your desired minimum number of nights. Then click on "Save" and on "Save" again.
Steve.