You will have to sign up for the royal Perks program, and I assume this is not YMMV.
The free fries will be added each Monday.
Royal Perks Members will receive 1 free fry (with purchase), per week for the rest of 2?0?2?2. Account registration required. Must sign up before J?u?n?e 2?0, 2?0?2?2. Offer valid from A?p?r?i?l 2?5, 2?0?2?2 to D?e?c?e?m?b?e?r 3?1, 2?0?2?2 at participating U.S. Burger King® restaurants, on the BK® App and BK.com. Limit one per week/account. Not valid on delivery orders. Must add offer to cart before placing order. Not valid with any other coupons or offers. Not valid with Mix & Match, Mix & Match Meals, and Your Way Meal. Not valid in AK, HI, and U.S. territories. Offer expires every S?u?n?d?a?y at 1?1?:5?9 PM ET. A new offer will be added each Monday for the duration of the term. No cash value or substitutions. Royal Perks terms apply. See bk.com/rewards-terms.