@Mandy575
There is no way to pre-set a Special Offer, as the option only appears when a guest sends a message and inquires about a stay. You can mention in your description that you will extend a special offer (I would use the word "Discount") for specific dates.

A better way to do this is still mention in the description your discount and then use Professional Tools and a Rule Set to automatically provide the discount for a range of dates without using a special offer. Sending Special Offers do run the risk of errors when you are calculating the discounts and sometimes Hosts forget to add the cleaning fee (or other fees) to the special offer - Airbnb won't do that for you on a special offer; they only add their fee and taxes.
Be sure you double check your Rule Set is working properly by trying to book as a guest (they can be tricky). You can only see the multi-calendar on a laptop (not in the App).
Using Professional Hosting Tools
https://www.airbnb.com/help/article/2499
Special Offers
https://www.airbnb.com/help/article/35