If you know who you are offering the free class to -> You can just add those people into the class by going into the class ->Calendar -> Attendance, click the "+" to add in those clients you wish to add and they will added to the class for free.
Offer a Discount for a drop-in session -> Client Billing -> Create a Discount for the amount to drop in class is worth -> You can now share the discount code with your clients/new prospects OR
you can edit your drop-in package and make the Coupon Code Field Show (make sure it is highlighted green) and they can apply the coupon when signing up for the class.
Create a Free Class -> You will need to create a new service item, you may want to call it Free Drop-In -> When creating the class in Service Items, make sure you do not attach it to any memberships or packages so it will truly be free. You will then schedule the class within your calendar for a date and time. If you create a free class and have it public EVERYONE will be able to take this class. You can make it private and send the reservation link to your new prospects so they can sign up-> You will went to let them know they will find their reservation (once signed up) in "Your Schedule" under their drop down menu in Client Portal.