You can configure a listing's settings in Guesty to ensure that it reflects as a "hostel" in Booking.com. Follow the instructions below.
If the listing is a multi-unit, this setup should be managed via the multi-unit's main settings, rather than via a sub-unit's settings. The settings will automatically apply to all sub-units. Learn more.
Step by step:
Sign in to your Guesty account.
In the top menu, click Listings.
Click the relevant listing.
In the listing's menu, click Property.
Under Listing Type, select Shared room from the drop-down.
Under Property Type, select Dorm from the drop-down.
Under Room Type, select Dormitory room from the drop-down.