Not sure what you mean by "granted retroactively". In any event, if you are claiming a credit for a 2015 expenditure, you'll need to amend your 2015 tax return. See here for help with amending a prior tax return:
https://ttlc.intuit.com/questions/3410543-how-do-i-amend-change-or-correct-a-2015-return-in-turbotax
