How does multiclassing affect the damage scaling of Hexblade’s curse? [duplicate]

So,I am playing a multiclass character with level 14 in fighter and level 1 in Warlock. Now Hexblade’s curse damage depends on player’s proficiency bonus.My level 15 character’s proficiency bonus is +5 but a level1 warlock’s proficiency bonus is +2. So, which proficiency bonus should I add as hexblade’s curse damage?