You cannot do this by clicking afterburn, but if you hit '3' it goes through and allows you to imbue the ability.
From my tests, this seems to apply a permanent spirit power buff on the afterburn.
Also, it seems that imbuing surge of power on mirage's trait buffs the passive even if you don't activate it, and I don't believe this is intended. I tested it in the following way: When I apply surge of power to mirage's 3 and hit the target dummy with 1 stack but I do not activate it, it does 37 damage. Surge of power's effect shouldn't trigger. However, when I compare it to an ability of the same tier of spirit item but with no extra spirit buff - if I buy silence glyph instead, for the same +12 base spirit as surge - 1 stack of the passive does 19 damage.
So I'm pretty sure applying surge to mirage's 3 boosts both the passive and also has an effect when you activate it. It gives no visual indicator of this however.
From my tests, this seems to apply a permanent spirit power buff on the afterburn.
Also, it seems that imbuing surge of power on mirage's trait buffs the passive even if you don't activate it, and I don't believe this is intended. I tested it in the following way: When I apply surge of power to mirage's 3 and hit the target dummy with 1 stack but I do not activate it, it does 37 damage. Surge of power's effect shouldn't trigger. However, when I compare it to an ability of the same tier of spirit item but with no extra spirit buff - if I buy silence glyph instead, for the same +12 base spirit as surge - 1 stack of the passive does 19 damage.
So I'm pretty sure applying surge to mirage's 3 boosts both the passive and also has an effect when you activate it. It gives no visual indicator of this however.