Tasha’s Druid Circle of the Stars Cosmic Omen

I have a question about Circle of the Stars 6th level feature:

Cosmic Omen
When you reach 6th level, you learn to use your star map to divine the will of the cosmos. Whenever you finish a long rest, you can consult your Star Map for omens. When you do so, roll a die. Until you finish your next long rest, you gain access to a special reaction based on whether you rolled an even or an odd number on the die:
Weal (even). Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to roll a d6 and add the number rolled to the total.
Woe (odd). Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to roll a d6 and subtract the number rolled from the total.
You can use this reaction a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

I’m not sure whether I have to use my reaction before I know the score the enemy/party member rolled or I can do it after I know what their score is.