As a reaction, which you use when you make a Will saving throw, you fortify your mind against intrusions. You make the roll with advantage. You can do so after the roll but before any effects of the roll occur.

If the roll already has advantage or disadvantage, you cannot apply another instance of advantage or disadvantage and you must use this before the roll to cancel out any advantage or disadvantage.

Augment

You can expend 2 additional mana to trigger the spell when you or one of your allies within 2 meters of you makes a Will saving throw. Until the start of your next turn, you and all allies within 2 meters of you have advantage on Will saving throws.