Adding it after fermentation will deliver a better flavour and aroma profile, however, not doing so wouldn't warrant throwing out the cider.
Generally speaking, adding flavours at the beginning of fermentation will give a more subtle, rustic, and fermented fruit flavour compared to adding at the end which will give a fresher, fuller, and brighter character.
So, the brew should still turn out well but just with the flavour and aroma being slightly less strong than intended.