@Fargol0
A long term reservation is a reservation for 28 or more nights. It does not matter on which dates such a period starts or ends. The "long term cancellation policy" is automatically applied. So you can not "set" yourself a reservation as a "long term", it's simply done by Airbnb if it meets the 28 nights or more condition.
Read more:
https://www.airbnb.com/help/article/1354/what-are-some-differences-between-long-term-hosting-and-sho...
https://www.airbnb.com/help/article/1361/how-do-changes-and-cancellations-work-for-long-term-reserva...
If you want to set a certain period on your calender only available for those 'long term' bookings, then add a ruleset to that period (minimum 28 days stay) .
Ruleset are available when the multicalender is displayed. Please note you also must set a monthly discount, to be found in the search system for "long term stays"
if no multicalender availabe, there is a similar setting in the section "availibilty" at "trip lenght" choose EDIT, then choose "add a rule for seasons or specific dates." and select the custom period which you want to set to a "28 nights or more minimum stay".
Best regards,
Emiel